numpy amax (aka max) doesn't get passed axis parameter #69

this is also mentioned in #41.

expected:

>>> a = [[1, -2, -3], [0, 0, 0]] * pq.m
>>> a.max(axis=0)
array([1, 0, 0]) * meters

observed: it always returns the max of all of the elements, no matter what axis you specify.

numpy.amin (aka min) also has the same issue.

Thanks!