gcd(): allow user to override (some of) heuristics.

gcd(): allow user to override (some of) heuristics.

GiNaC tries to avoid expanding expressions while computing GCDs and applies
a number of heuristics. Usually this improves performance, but in some cases
it doesn't. Allow user to switch off heuristics.

Part 5:

* gcd(): don't use heuristic GCD algorithm if gcd_options::no_heur_gcd
  flag is set.
* gcd(): don't handle partially factored expressions in a special way
  if gcd_options::no_part_factored flag is set.