Refactor verifyproblem - extract Diagnostics #398

[gkreitz]

Starting to break apart verifyproblem, #398. These PR:s are going to quickly end up on the large side, unfortunately.

This PR cleans up the interface to Problem a bit by doing two things:

• Extracts an observer, called Diagnostics which a called to Problem supplies and which handles logging and error counting. This got rid of most of the reasons for Problem taking an argparse.Namespace argument
• Moved the remaining config (parts to check, number of threads) to Context, and let the caller supply that when calling problem.check()

The Diagnostics objects are currently stored in ProblemAspect, to keep the size of this PR a bit more manageable.