add function cumulative rainflow spectrum

[kchong75]

@rpauly18 @hivanov-nrel
PR for mhkit-fatigue: cumulative rainflow spectrum

[rpauly18]

I would add an assert statement checking to see if this value is above 100

[ssolson]

Hey @rpauly18 we are no longer using assert statements. See #242.

For the current assert the replacement would be:

    if not isinstance(bin_num, (float, int)):
        raise ValueError('bin_num must be of type float or int')

ChatGPT is pretty good at converting these but we need to always check its work.

[rpauly18]

@hivanov-nrel can you take a look at this and make sure the functionality is as expected? Can you also develop a quick test for this?