No API / function to plot a histogram with scores in memory

There is no way of creating a score histogram (like bob bio hist from bob.bio.base) from code without saving the scores to a file.

det, roc and other plots commands have a function that allows feeding negative and positive scores as lists, but hist does all the plotting in the Figure class.

We should consider moving the plotting part of the bob.measure.script.figure.Hist class in an hist function in the bob.measure.plot module.