Fixed no parameter changes when max_radii and cutoffs not used.
Fixed no parameter changes when max_radii and cutoffs not used.

When any method in gradient generates parameter changes, these
are then run through gradient.check, which checks whether the radius
of unscaled parameter changes meets certain user specified conditions
(currently, those conditions are given in the __init__ statement of
gradient.Gradient).

It would first check for max_radii and then for cutoffs. However,
if neither were used, it would incorrectly return None rather than
returning the unscaled parameter changes.

This should fix issue #7.

arosale4/q2mm,ericchansen/q2mm,arosale4/q2mm,Q2MM/q2mm,Q2MM/q2mm,ericchansen/q2mm