Various improvements in finance.py #3094

While working on #3060, I've added a private function to check the input of some other functions.

I suggest:

• Improve (some) docstring: e.g. in some function there is information about the input structure in the code comments and this should be moved into the docstring
• Avoid using buildin for variable names
• Improve loops: e.g. avoid use zip(xrange(len(a)), a, ...) when zip(itertools.count(), a, ...) or enumerate(a) would suffice