`func_poly2d` always returns `None`

[AlexKurek]

What is the purpose of computing z in this function?
https://github.com/lofar-astron/PyBDSF/blob/master/bdsf/functions.py#L68
Becouse of:
z = None
https://github.com/lofar-astron/PyBDSF/blob/master/bdsf/functions.py#L87
it will always return None. Looks like an indentation bug.

[gmloose]

@darafferty, it looks like neither func_poly2d nor func_poly2d_ini are used by PyBDSF. Do you know if they're used outside PyBDSF? If not, we might as well complete remove those functions.

[darafferty]

Hmm, indeed. I'm not aware of any usage of them outside of PyBDSF, so I agree they should be removed.

[AlexKurek]

In that case, I have modified the relevant PR. The tests were passed without these two functions.

[AlexKurek]

• Also flatten(x) seems not to be needed. It has basestring inside, with is not avalilable in Python 3.
• polar2cart is unused also. It has missing numpy import inside, it would crash immediately.