Zoom always does a partial=True type match

[kpeeters]

For convenience, the zoom algorithm always allows for more than the indicated factors in any term:

ex:=A D + A B C D E;
zoom(_, $A D$);

keeps both terms. For backward compatibility we cannot change this, but it would be useful to have a partial=False type flag which forces zoom to only keep the first term in the example above (similar to what substitute does).