keep I in Factors

This is a little troubling since transform tests fail if the I is
removed from the Factors factors and replaced with sqrt(-1).
AppliedUndef.is_number -> False
exp_polar related changes

* define is_number for exp_polar

* move abs after test for exp_polar (which has been changed to a test
that arg.is_Number or pure_complex rather than testing for the _prec i
attribute).
add quick exit for simplify if expr is Atom
modify equals and is_constant