Generalize ReagentUnit into FixedPoint2 and use it for damage calculations (#5151)

* Damage units

* sum ext method

Content.IntegrationTests/Tests/Chemistry/FixedPoint2SerializationTest.cs

@@ -0,0 +1,44 @@
+using System.Reflection;
+using Content.Shared.Chemistry.Reagent;
+using Content.Shared.FixedPoint;
+using NUnit.Framework;
+using Robust.Shared.Serialization.Manager;
+using Robust.Shared.Serialization.Manager.Attributes;
+using Robust.Shared.Serialization.Markdown.Mapping;
+using Robust.Shared.Serialization.Markdown.Value;
+using Robust.UnitTesting.Shared.Serialization;
+
+namespace Content.IntegrationTests.Tests.Chemistry
+{
+    public class FixedPoint2SerializationTest : SerializationTest
+    {
+        protected override Assembly[] Assemblies => new[]
+        {
+            typeof(FixedPoint2SerializationTest).Assembly
+        };
+
+        [Test]
+        public void DeserializeNullTest()
+        {
+            var node = new ValueDataNode("null");
+            var unit = Serialization.ReadValue<FixedPoint2?>(node);
+
+            Assert.That(unit, Is.Null);
+        }
+
+        [Test]
+        public void DeserializeNullDefinitionTest()
+        {
+            var node = new MappingDataNode().Add("unit", "null");
+            var definition = Serialization.ReadValueOrThrow<FixedPoint2TestDefinition>(node);
+
+            Assert.That(definition.Unit, Is.Null);
+        }
+    }
+
+    [DataDefinition]
+    public class FixedPoint2TestDefinition
+    {
+        [DataField("unit")] public FixedPoint2? Unit { get; set; } = FixedPoint2.New(5);
+    }
+}