Add pound per pound-mole molar mass (#1703)

## Motivation

Fixes #1668.

`PoundPerMole` currently means pound mass per SI mole, `lb/mol`. That is
a valid unit and I don't think we should silently redefine it.

The issue is that engineering and manufacturing users often need
molecular weight in English Engineering units, where the common unit is
pound mass per pound-mole, `lb/lbmol`. That should be numerically equal
to `kg/kmol`, but it is not the same as `lb/mol`.

## Changes

- Add `MolarMassUnit.PoundPerPoundMole` with `lb/lbmol` and `lbm/lbmol`
abbreviations.
- Use `Pound` and the existing `PoundMole` amount-of-substance unit as
base units.
- Keep `PoundPerMole` unchanged, but document that it means pound mass
per SI mole.
- Regenerate UnitsNet and number-extension APIs.
- Add tests showing `lb/lbmol` matches `kg/kmol` and remains distinct
from `lb/mol`.

## Validation

- `generate-code.bat`
- `dotnet test UnitsNet.Tests --filter
"FullyQualifiedName~UnitsNet.Tests.CustomCode.MolarMassTests"`

Author: angularsen

Common/UnitDefinitions/MolarMass.json

@@ -47,6 +47,7 @@
     {
       "SingularName": "PoundPerMole",
       "PluralName": "PoundsPerMole",
+      "XmlDocSummary": "Pound mass per SI mole. This is distinct from PoundPerPoundMole, where the denominator is the pound-mole amount of substance unit.",
       "BaseUnits": {
         "M": "Pound",
         "N": "Mole"
@@ -64,6 +65,24 @@
           "Abbreviations": [ "фунт/моль" ]
         }
       ]
+    },
+    {
+      "SingularName": "PoundPerPoundMole",
+      "PluralName": "PoundsPerPoundMole",
+      "XmlDocSummary": "Pound mass per pound-mole. This is numerically equal to kilograms per kilomole and distinct from PoundPerMole, where the denominator is the SI mole.",
+      "XmlDocRemarks": "Sources: https://www.engineeringtoolbox.com/molecular-weight-gas-vapor-d_1156.html, https://www.engineeringtoolbox.com/unit-converter-d_185.html",
+      "BaseUnits": {
+        "M": "Pound",
+        "N": "PoundMole"
+      },
+      "FromUnitToBaseFunc": "{x} / 1e3",
+      "FromBaseToUnitFunc": "{x} * 1e3",
+      "Localization": [
+        {
+          "Culture": "en-US",
+          "Abbreviations": [ "lb/lbmol", "lbm/lbmol" ]
+        }
+      ]
     }
   ]
 }

Common/UnitEnumValues.g.json

@@ -1055,7 +1055,8 @@
     "NanogramPerMole": 11,
     "PoundPerMole": 12,
     "KiloGramPerKiloMole": 20,
-    "KilogramPerKilomole": 15
+    "KilogramPerKilomole": 15,
+    "PoundPerPoundMole": 24
   },
   "Permeability": {
     "HenryPerMeter": 1

UnitsNet.NumberExtensions.CS14.Tests/GeneratedCode/NumberToMolarMassExtensionsTest.g.cs

@@ -41,8 +41,32 @@ public class MolarMassTests : MolarMassTestsBase
         protected override double NanogramsPerMoleTolerance => 1e-3;
         protected override double NanogramsPerMoleInOneKilogramPerMole => 1e12;
         protected override double PoundsPerMoleInOneKilogramPerMole => 2.2046226218487757;
+        protected override double PoundsPerPoundMoleInOneKilogramPerMole => 1e3;
         protected override double KilogramsPerKilomoleInOneKilogramPerMole => 1e3;
 
+        [Fact]
+        public void PoundPerPoundMoleMatchesKilogramPerKilomole()
+        {
+            // Engineering Toolbox lists air molecular weight as 28.966 in kg/kmol, g/mol, and lb/lbmol.
+            const double airMolecularWeight = 28.966;
+
+            MolarMass molarMass = MolarMass.FromPoundsPerPoundMole(airMolecularWeight);
+
+            Assert.Equal(airMolecularWeight, molarMass.KilogramsPerKilomole, precision: 12);
+        }
+
+        [Fact]
+        public void PoundPerPoundMoleIsDistinctFromPoundPerMole()
+        {
+            const double value = 1;
+
+            MolarMass poundPerPoundMole = MolarMass.FromPoundsPerPoundMole(value);
+            MolarMass poundPerMole = MolarMass.FromPoundsPerMole(value);
+
+            Assert.Equal(value, poundPerPoundMole.KilogramsPerKilomole, precision: 12);
+            Assert.True(poundPerMole.KilogramsPerKilomole > poundPerPoundMole.KilogramsPerKilomole);
+        }
+
         [Fact]
         public void MolarMassTimesAmountOfSubstanceEqualsMass()
         {