Add a construct_ffd() function

[ekaterinailin]

What needs to be created or improved?

Implement a function that creates a cumulative FFD object given a set of true or fake flares detected in one or multiple light curves.

Can you provide an example?

@test_units
def construct_ffd(flares, **kwargs):
    hist, bins = np.cumhist(flares.ed)
    [...]
    return ffd

flares = lc.find_flares()
ffd = construct_ffd(flares)

What is the goal / expected behaviour?

Create a simple FFD with frequency over ED for a quick view of flare analysis results that can be easily extended to include real energies (quiescent luminosity provided), log-log representation, power law fits and their uncertainties.

• [x] open issues on the additional enhancements mentioned above.

[ekaterinailin]

This is implemented and documented now: https://altaipony.readthedocs.io/en/latest/api/ffds.html