gh2666: terms_gcd does not remove 1.0

- [x] remove changes to docstrings that bled over from other PR

In addition, if only a 1 (or 1.0) is extracted, the original
expression is returned from terms_gcd rather than the polys-modified
form (e.g. 2.0*x + 3 instead of 2.0*x + 3.0).

fixes gh-2666