Make GasMixture enumerable

I noticed that enumerating gases is frequently done in an annoying way with Enum.GetValues. So I made it better. Now GasMixture is IEnumerable<(Gas gas, float moles)> and it just works.

Content.Tests/Shared/Atmos/GasMixtureTest.cs

@@ -0,0 +1,30 @@
+using Content.Shared.Atmos;
+using NUnit.Framework;
+
+namespace Content.Tests.Shared.Atmos;
+
+[TestFixture, TestOf(typeof(GasMixture))]
+[Parallelizable(ParallelScope.All)]
+public sealed class GasMixtureTest
+{
+    [Test]
+    public void TestEnumerate()
+    {
+        var mixture = new GasMixture();
+        mixture.SetMoles(Gas.Oxygen, 20);
+        mixture.SetMoles(Gas.Nitrogen, 10);
+        mixture.SetMoles(Gas.Plasma, 80);
+
+        var expectedList = new (Gas, float)[Atmospherics.TotalNumberOfGases];
+        for (var i = 0; i < Atmospherics.TotalNumberOfGases; i++)
+        {
+            expectedList[i].Item1 = (Gas)i;
+        }
+
+        expectedList[(int)Gas.Oxygen].Item2 = 20f;
+        expectedList[(int)Gas.Nitrogen].Item2 = 10f;
+        expectedList[(int)Gas.Plasma].Item2 = 80f;
+
+        Assert.That(mixture, Is.EquivalentTo(expectedList));
+    }
+}