Array function interface not triggered on scalars

[mocquin]

Calling a numpy function with only scalars will not trigger the array function interface, since it is used only when an argument is an array.

from physipy import m
# this raises a DimensionError because of the casting into float
#np.random.normal(3*m, 1*m)
# while this works but return a float
np.random.normal(np.array(3*m), np.array(1*m))

# this also drops the unit
np.sum(np.array(3*m))