Figure.histogram: Let parameter 'cumulative' support Pythonic arguments

[seisman]

Migrate the parameter cumulative to the new alias system.

Previously, cumulative can be either True or "r". This PR lets it support more Pythonic argument, i.e., cumulative="reversed".

For comparison, here is matplotlib's hist method (https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.hist.html):

cumulativebool or -1, default: False
If True, then a histogram is computed where each bin gives the counts in that bin plus all bins for smaller values. The last bin gives the total number of datapoints.
If density is also True then the histogram is normalized such that the last bin equals 1.
If cumulative is a number less than 0 (e.g., -1), the direction of accumulation is reversed. In this case, if density is also True, then the histogram is normalized such that the first bin equals 1.

So, matplotlib uses cumulative=-1 for reversed cumulative histogram, but I feel it's not as readable as cumulative="reversed".

Preview: