Add NewtonsPerMillimeter to ForcePerLength (#743)

Author: ajphall

Common/UnitDefinitions/ForcePerLength.json

@@ -19,6 +19,18 @@
           "Abbreviations": [ "N/m" ]
         }
       ]
+    },
+	{
+      "SingularName": "NewtonPerMillimeter",
+      "PluralName": "NewtonsPerMillimeter",
+      "FromUnitToBaseFunc": "x*1e3",
+      "FromBaseToUnitFunc": "x/1e3",
+      "Localization": [
+        {
+          "Culture": "en-US",
+          "Abbreviations": [ "N/mm" ]
+        }
+      ]
     },
     {
       "SingularName": "KilogramForcePerMeter",

UnitsNet.Tests/CustomCode/ForcePerLengthTests.cs

@@ -31,6 +31,7 @@ public class ForcePerLengthTests : ForcePerLengthTestsBase
         protected override double MillinewtonsPerMeterInOneNewtonPerMeter => 1E3;
         protected override double NanonewtonsPerMeterInOneNewtonPerMeter => 1E9;
         protected override double NewtonsPerMeterInOneNewtonPerMeter => 1;
+        protected override double NewtonsPerMillimeterInOneNewtonPerMeter => 1E-3;
         protected override double PoundsForcePerFootInOneNewtonPerMeter => 6.8521766E-2;
         protected override double PoundsForcePerInchInOneNewtonPerMeter => 5.710147162769201E-3;
         protected override double PoundsForcePerYardInOneNewtonPerMeter => 0.205565298;