Rename occurences of 'MyUnit' to 'MyQuantity' in generated headers

[angularsen]

This is a remnant of an earlier rename from "units" to "quantities".

Example:

UnitsNet/UnitsNet/GeneratedCode/Units/AmountOfSubstanceUnit.g.cs

Line 11 in 6e23232

// Add CustomCode\Quantities\MyUnit.extra.cs files to add code to generated quantities.

Fix this by both updating all .cs files (some are only generated once) and the .ps1 powershell code generator files, then running generate_code.bat.