QuantityFormatter defaults to the "G" format (#1450)

- [x] `QuantityFormatter` defaults to the "G" format for null or empty
string
- [x] `QuantityFormatter` no longer supports the `U`/`V`/`Q` formats (an
exception is thrown)
- [x] `QuantityFormatter` explicitly throws for the `Cx`/`Px` formats
- [x] `QuantityFormatter` shouldn't throw for something like "P1: #.00"
(instead it should output something like "P1: 12.34 mg")
- [x] `QuantityFormatter` unless explicitly given an incorrect `Ax`
specifier, the `QuantityFormatter` uses the "default unit abbreviation"
- which maybe `string.Empty` (if none are defined for a given unit)

Fixes #1183 
Duplicate of #1206

CC @tmilnthorp

---------

Co-authored-by: Andreas Gullberg Larsen <[email protected]>

Author: lipchev

CodeGen/Generators/UnitsNetGen/QuantityGenerator.cs

@@ -1170,7 +1170,7 @@ private void GenerateToString()
         /// <returns>String representation.</returns>
         public override string ToString()
         {{
-            return ToString(""g"");
+            return ToString(null, null);
         }}
 
         /// <summary>
@@ -1180,7 +1180,7 @@ public override string ToString()
         /// <param name=""provider"">Format to use for localization and number formatting. Defaults to <see cref=""CultureInfo.CurrentCulture"" /> if null.</param>
         public string ToString(IFormatProvider? provider)
         {{
-            return ToString(""g"", provider);
+            return ToString(null, provider);
         }}
 
         /// <inheritdoc cref=""QuantityFormatter.Format{{TUnitType}}(IQuantity{{TUnitType}}, string, IFormatProvider)""/>
@@ -1191,7 +1191,7 @@ public string ToString(IFormatProvider? provider)
         /// <returns>The string representation.</returns>
         public string ToString(string? format)
         {{
-            return ToString(format, CultureInfo.CurrentCulture);
+            return ToString(format, null);
         }}
 
         /// <inheritdoc cref=""QuantityFormatter.Format{{TUnitType}}(IQuantity{{TUnitType}}, string, IFormatProvider)""/>
@@ -1277,7 +1277,7 @@ float IConvertible.ToSingle(IFormatProvider? provider)
 
         string IConvertible.ToString(IFormatProvider? provider)
         {{
-            return ToString(""g"", provider);
+            return ToString(null, provider);
         }}
 
         object IConvertible.ToType(Type conversionType, IFormatProvider? provider)

CodeGen/Generators/UnitsNetGen/UnitTestBaseClassGenerator.cs

@@ -662,7 +662,7 @@ public void ToString_NullFormat_DefaultsToGeneralFormat(string cultureName)
                 ? null
                 : CultureInfo.GetCultureInfo(cultureName);
 
-            Assert.Equal(quantity.ToString(""g"", formatProvider), quantity.ToString(null, formatProvider));
+            Assert.Equal(quantity.ToString(""G"", formatProvider), quantity.ToString(null, formatProvider));
         }}
 
         [Theory]