Add BaggingClassifier and BaggingRegressor support

[desilinguist]

• Fix: base estimators cannot be serialized
  • It turns out that there's a long standing bug in SKLL where using meta-estimators like AdaBoost* that use a base estimator are unable to be used with run_experiment since their base estimator prevents the model parameters dictionary from being serialized to disk. Added code that replaces the base estimator object with a string before serializing.
  • Existing tests did not cover this scenario since they used those meta-estimators via the API and not via a configuration file. Added new tests that covered this scenario.
• Add support for BaggingClassifier and BaggingRegressor
  • Enable them to be used in SKLL.
  • Set default parameter grids for grid search.
• Add documetnation for Bagging{Classifier,Regressor} use.
• New tests for Bagging{Classifier,Regressor}

Easiest way to review is to try out the new learners (along with different estimator fixed parameter values) in the California and Titanic examples.

This PR closes #614.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (ce40d5b) 95.19% compared to head (0108e7b) 95.19%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #742 +/- ## ======================================= Coverage 95.19% 95.19% ======================================= Files 29 29 Lines 3535 3538 +3 ======================================= + Hits 3365 3368 +3 Misses 170 170 ``` | [Impacted Files](https://app.codecov.io/gh/EducationalTestingService/skll/pull/742?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService) | Coverage Δ | | |---|---|---| | [skll/utils/constants.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/742?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC91dGlscy9jb25zdGFudHMucHk=) | `100.00% <ø> (ø)` | | | [skll/experiments/utils.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/742?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9leHBlcmltZW50cy91dGlscy5weQ==) | `93.54% <100.00%> (+0.15%)` | :arrow_up: | | [skll/learner/\_\_init\_\_.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/742?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9sZWFybmVyL19faW5pdF9fLnB5) | `97.18% <100.00%> (ø)` | |

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