ENH : bin_edeges helper function

Adds helper function + tests for generating bin edges from
any sub-set of the maximum, minimum, step, and number of bins.
if they are not numbers of some sort, this will blow up with an error (which is the job of the duck-typing to raise, not ours).  Checking for floats would make this not work with integers.

In some sense, it is done at a lower level.