simplify checks for time varying solveOnePeriod, fixes #496

[sbenthall]

This PR resolves #496 by simplifying the code around multiple solveOnePeriod methods:

• The check is performed in only one place (less overhead on user reading and understanding the code)
• It does not require special use of the time_vary list (less overhead on user building on the code)

[sbenthall]

Regarding automated tests for the functionality touched by this PR:

There are two cases:

• The first case, where solveOnePeriod is a function, is tested by this test: https://github.com/econ-ark/HARK/blob/master/HARK/tests/test_core.py#L100
• The second case, where solveOnePeriod is subscriptable:
  • has never been used
  • there appears to be no example in the literature of how it would be used
  • theoretically, if there were a case where it could be used, it would be possible to implement it otherwise by having solveOnePeriod be a function that had several cases.

I'm hesitant to write a test for this functionality that simply may not be necessary or desirable in the first place.