Feature Request: Add support for GP as single currency

Add support for the common home rule of treating all currency as some value of Gold Pieces (decimal value down to equivalent in Copper Pieces)

Additionally, subtracting currency values for the current currency model - the current implementation does not automatically attempt to change currency types down in value, causing the issue that you can attempt to subtract more of a given currency type than you have and you simply hit zero for that piece type with no indicated error.