implemented more usful determine_n_fft function
added fft units aware
ContinuousDataEven has fft method, and another method to calculate it's
parameters of frequencies
taking fft relevant funcionality out of continuos/math.py
chenged the interface (and name) of determine_n_fft
and also changed the interface of fft in pint_extension
adjusted the tests to the refactor
changed the interface of ContinuousDataEven.fft
added test for ContunuousDataEven.fft
The tests work!