Add unit model and unit tests

MultiversalDiplomacy/Model/Power.cs

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+namespace MultiversalDiplomacy.Model;
+
+/// <summary>
+/// One of the rival nations vying for control of the map.
+/// </summary>
+public class Power
+{
+    /// <summary>
+    /// The power's name.
+    /// </summary>
+    public string Name { get; }
+
+    public Power(string name)
+    {
+        this.Name = name;
+    }
+}

MultiversalDiplomacy/Model/Unit.cs

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+namespace MultiversalDiplomacy.Model;
+
+/// <summary>
+/// Represents a unit at a particular point in time.
+/// </summary>
+public class Unit
+{
+    /// <summary>
+    /// The previous iteration of a unit. This is null if the unit was just built.
+    /// </summary>
+    public Unit? Past { get; }
+
+    /// <summary>
+    /// The location on the map where the unit is.
+    /// </summary>
+    public Location Location { get; }
+
+    /// <summary>
+    /// The season in time when the unit is.
+    /// </summary>
+    public Season Season { get; }
+
+    /// <summary>
+    /// The allegiance of the unit.
+    /// </summary>
+    public Power Power { get; }
+
+    /// <summary>
+    /// The type of unit.
+    /// </summary>
+    public UnitType Type { get; }
+
+    private Unit(Unit? past, Location location, Season season, Power power, UnitType type)
+    {
+        this.Past = past;
+        this.Location = location;
+        this.Season = season;
+        this.Power = power;
+        this.Type = type;
+    }
+
+    /// <summary>
+    /// Create a new unit. No validation is performed; the adjudicator should only call this
+    /// method after accepting a build order.
+    /// </summary>
+    public static Unit Build(Location location, Season season, Power power, UnitType type)
+        => new Unit(past: null, location, season, power, type);
+
+    /// <summary>
+    /// Advance this unit's timeline to a new location and season.
+    /// </summary>
+    public Unit Next(Location location, Season season)
+        => new Unit(past: this, location, season, this.Power, this.Type);
+}

MultiversalDiplomacy/Model/UnitType.cs

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+namespace MultiversalDiplomacy.Model;
+
+/// <summary>
+/// The type of a unit.
+/// </summary>
+public enum UnitType
+{
+    /// <summary>
+    /// A unit that moves on land.
+    /// </summary>
+    Army = 0,
+
+    /// <summary>
+    /// A unit that moves in oceans and along coasts and can convoy armies.
+    /// </summary>
+    Fleet = 1,
+}

MultiversalDiplomacyTests/TestMap.cs

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+using MultiversalDiplomacy.Map;
+using MultiversalDiplomacy.Model;
+
+namespace MultiversalDiplomacyTests;
+
+public class TestMap : Map
+{
+    public override IEnumerable<Province> Provinces { get; }
+
+    public static TestMap Instance { get; } = new TestMap();
+
+    private TestMap()
+    {
+        Provinces = new List<Province>()
+        {
+            Province.Supply("Left", "LEF")
+                .AddLandLocation(),
+            Province.Supply("Right", "RIG")
+                .AddLandLocation(),
+            Province.Empty("Center", "CEN")
+                .AddLandLocation(),
+        };
+        Land("LEF").AddBorder(Land("RIG"));
+        Land("LEF").AddBorder(Land("CEN"));
+
+        Land("RIG").AddBorder(Land("LEF"));
+        Land("RIG").AddBorder(Land("CEN"));
+
+        Land("CEN").AddBorder(Land("LEF"));
+        Land("CEN").AddBorder(Land("RIG"));
+    }
+}

MultiversalDiplomacyTests/UnitTests.cs

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+using MultiversalDiplomacy.Map;
+using MultiversalDiplomacy.Model;
+
+using NUnit.Framework;
+
+namespace MultiversalDiplomacyTests;
+
+public class UnitTests
+{
+    [Test]
+    public void MovementTest()
+    {
+        Map map = TestMap.Instance;
+        Location left = map.Land("LEF"), right = map.Land("RIG"), center = map.Land("CEN");
+        Power pw1 = new Power("First");
+        Season s1 = Season.MakeRoot();
+        Unit u1 = Unit.Build(left, s1, pw1, UnitType.Army);
+
+        Season s2 = s1.MakeNext();
+        Unit u2 = u1.Next(right, s2);
+
+        Season s3 = s2.MakeNext();
+        Unit u3 = u2.Next(center, s3);
+
+        Assert.That(u3.Past, Is.EqualTo(u2), "Missing unit past");
+        Assert.That(u2.Past, Is.EqualTo(u1), "Missing unit past");
+        Assert.That(u1.Past, Is.Null, "Unexpected unit past");
+
+        Assert.That(u1.Season, Is.EqualTo(s1), "Unexpected unit season");
+        Assert.That(u2.Season, Is.EqualTo(s2), "Unexpected unit season");
+        Assert.That(u3.Season, Is.EqualTo(s3), "Unexpected unit season");
+
+        Assert.That(u1.Location, Is.EqualTo(left), "Unexpected unit location");
+        Assert.That(u2.Location, Is.EqualTo(right), "Unexpected unit location");
+        Assert.That(u3.Location, Is.EqualTo(center), "Unexpected unit location");
+    }
+}