refactoring to use classes instead of strings for _REQUIRES_DENSE, _DEFAULT_PARAM_GRIDS.  replaced _REGRESSION_MODELS with check for RegressorMixin.
removed the now-unnecessary fixed_parameters setting from test_logistic_custom_learner.template.cfg
Update backward compatibility test to use 1.0.0 model format.
Fix path issues with new models for backward compat test.
Fixed predictions file for backward compatibility test.
removed out-of-date comment
Revert "Fixed predictions file for backward compatibility test."

This reverts commit 6b86ebac69157ab2ecc88213b65e63cfa3afb9ab.
Revert "Fix path issues with new models for backward compat test."

This reverts commit 9e9be69ef6dc2b3251c2751ec0e526b713f32592.
Revert "Update backward compatibility test to use 1.0.0 model format."

This reverts commit 2205feb1ea605fd97271e780fb7b122b9981661a.
added line for backward compatibility in Learner.from_file, per suggestion from @dan-blanchard
updated version check in Learner.from_file to support old models that would still work
Merge pull request #187 from EducationalTestingService/feature/custom_learners

refactoring to use classes instead of strings in Learner.py