Fix subtract() function not working with parameter type other than Big

[andriwagner]

Added type check of the parameter of the subtract() function. (same as in add() function) Now subtract() works with a float as a parameter.

I had the problem that the minusEquals() function did not work with a float as a parameter. The problem was that in the subtract() function, the parameters were not converted into a Big before accessing the attribute "exponent" of the parameter.