Argument naming convention in pycthermopack wrappers

[ailoa]

In the next major version release, consider making the python interfaces more uniform. The argument for mol numbers is now sometimes called n, sometimes x, and sometimes z.

[vegardjervell]

I agree that we should be consistent on using x vs. z for methods that expect molar composition, but think we should keep n vs. z separate to indicate that some methods expect mol numbers, and return extensive variables, while others expect molar composition and return intensive variables.