Fix FAMoS termination; remove other switching methods

[dilpath]

The FAMoS candidate space returns no models, if one of its inner methods (e.g. the forward method) provides no models. Calibration tools (e.g. pyPESTO) then terminate the model selection, because no models were provided. However, at this point, FAMoS should switch to another method.

This switching is now implemented, in petab_select/ui.py.

The other changes are to remove the other switching methods (ForwardAndBackwardCandidateSpace and BidirectionalCandidateSpace), because these methods are probably not used by anyone, and would suffer from the same issue currently. StopIterations would need to be implemented in them, similarly to FamosCandidateSpace, when the search should truly terminate.

[codecov-commenter]

Codecov Report

Attention: 30 lines in your changes are missing coverage. Please review.

Comparison is base (f9a2a87) 60.95% compared to head (6dae816) 57.58%.

Files	Patch %	Lines
petab_select/ui.py	5.88%	16 Missing :warning:
petab_select/model.py	9.09%	10 Missing :warning:
petab_select/criteria.py	20.00%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #68 +/- ## =========================================== - Coverage 60.95% 57.58% -3.37% =========================================== Files 14 14 Lines 1534 1490 -44 =========================================== - Hits 935 858 -77 - Misses 599 632 +33 ```

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