Use readonly for structs in QuantityGenerator (#1135)

Fixes #1130

https://docs.microsoft.com/en-us/dotnet/csharp/write-safe-efficient-code#declare-immutable-structs-as-readonly

Force the compiler to check, that all fields are readonly as well.

Author: Brunni

CodeGen/Generators/UnitsNetGen/QuantityGenerator.cs

@@ -72,7 +72,7 @@ namespace UnitsNet
             Writer.WLIfText(1, GetObsoleteAttributeOrNull(_quantity));
             Writer.W(@$"
     [DataContract]
-    public partial struct {_quantity.Name} : IQuantity<{_unitEnumName}>, ");
+    public readonly partial struct {_quantity.Name} : IQuantity<{_unitEnumName}>, ");
             if (_quantity.BaseType == "decimal")
             {
                 Writer.W("IDecimalQuantity, ");