revisit train-test split

[mastoffel]

• splits into train/test set
• runs CV / hyperparam search on training set
• implements evaluate_model to get prediction metrics on a single model
• implements plot_model to plot test set predictions for a single model

resolves #161

[github-actions[bot]]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
autoemulate
compare.py					305, 338, 395
cross_validate.py
data_splitting.py
plotting.py					288-334
printing.py
tests
test_compare.py
test_cross_validate.py
test_data_splitting.py
test_printing.py
Project Total

This report was generated by python-coverage-comment-action

[codecov-commenter]

Codecov Report

Attention: 32 lines in your changes are missing coverage. Please review.

Comparison is base (28541d5) 91.62% compared to head (8caee01) 90.56%.

Files	Patch %	Lines
autoemulate/plotting.py	12.12%	29 Missing :warning:
autoemulate/compare.py	85.71%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #173 +/- ## ========================================== - Coverage 91.62% 90.56% -1.06% ========================================== Files 40 42 +2 Lines 1790 1834 +44 ========================================== + Hits 1640 1661 +21 - Misses 150 173 +23 ```

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