Rename loss => elementwise_loss and full_objective => loss_function

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Fix example code

Change default niterations to 40

Change PySR defaults; fixes #99

Update README example

Update docs

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Update README to use scikit-learn API

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Add __repr__ method that lists selected equation

Remove mistaken local Project.toml from example.py

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Add custom loss to example.py

Enable custom loss functions

Rename example file so it is obviously an example