Complete migration of GUI to run algorithms in threads

[tturocy]

This completes the migration of the GUI calls to use threads to run algorithms, by implementing the needed cancel-check points inside the remaining methods.

Closes #990.

[tturocy]

@d-kad @rahulsavani As before, exercise the GUI here on the rest of these methods!

[d-kad]

The tests I did:

• simpdiv for rand10, one eq: completes instantly
• ipa for rand10, some eq: completes instantly
• gnm for rand10, some eq: 5 profiles, including the pure (1:2, 2:5)
• enumpoly for rand3, one eq: matches gambit-enumpoly
• enumpoly for rand20, some eq: found 2 before I stopped -- responded immediately; GUI responsive throughout
• liap for rand6 and rand10, some eq: finds nothing, but gambit-liap -n 10 found nothing either