3693: combsimp adjustment for ratios
fix failing tests (confirmed equivalence numerically)
allow for iteration in rule_gamma
ExpBase.as_numer_denom synched with Pow

With this change, we no longer need to look for 1/(g-1) in numer nor
1/g in denom.
tidy up bsplines
make sure bottom_up goes all the way to the bottom
pep8 for fu.py
Merge pull request #1891 from smichr/comb

issue 3694: combsimp upgrades and bottom_up mods