Default window_type for power_spectral_density does not work

[DriesAllaerts]

In helper_functions.py>power_spectral_density(), the default window_type is 'hanning'. However, this window_type raises a ValueError "unknown window type". Accepted window types are either "hamming" or "hann". The term "hanning" window is a common mistake but it is not the official name and hence not accepted by the welch method.

Solution: Change "hanning" to "hann".