v0.8.0

This new release updates the entire set of default PySR parameters according to the ones presented in #115. These parameters have been tuned over nearly 71,000 trials. See the discussion for further info.

Additional changes:

• Nested constraints implemented. For example, you can now prevent sin and cos from being repeatedly nested, by using the argument: nested_constraints={"sin": {"sin": 0, "cos": 0}, "cos": {"sin": 0, "cos": 0}}. This argument states that within a sin operator, you can only have a max depth of 0 for other sin or cos. The same is done for cos. The argument nested_constraints={"^": {"+": 2, "*": 1, "^": 0}} states that within a pow operator, you can only have 2 things added, or 1 use of multiplication (i.e., no double products), and zero other pow operators. This helps a lot with finding interpretable expressions!
• New parsimony algorithm (backend change). This seems to help searches quite a bit, especially when one is searching for more complex expressions. This is turned on by use_frequency_in_tournament which is now the default.
• Many backend improvements: speed, bug fixes, etc.
• Improved stability of multi-processing (backend change). Thanks to @CharFox1.
• Auto-differentiation implemented (backend change). This isn't used by default in any instances right now, but could be used by optimization later. Thanks to @kazewong.
• Improved testing coverage of weird edge cases.
• All parameters to PySRRegressor have been cleaned up to be in snake_case rather than CamelCase. The backend is also now almost entirely snake_case for internal functions. +Other readability improvements. Thanks to @bstollnitz and @patrick-kidger for the suggestions.