Cmdstanpy

_Note_: This is a release candidate. If no issues are reported, the same version will be published as a full release in one week.

New Functionality
- Add method `log_prob` to model class (637)
- Extend `generate_quantities` to non-HMC classes (634)

Other changes
- Improve type hints (631)
- Serialize draws when pickled (632)
- Raise an error if `src_info` fails (638)
- Change tests to `pytest` format (639)

- `install_cmdstan` now downloads the correct CmdStan for non-x86 Linux machines (616).
- Improved reporting of errors during `compile()` (620).
- Fixed some edge cases in mixing arguments of the `optimize()` function (619).
- Fixed how `NaN` and infinite numbers were serialized to JSON (623).
- Removed dependency on `ujson`. For now, all JSON serialization is done with the Python standard library.
- Added a `timeout` parameter to all model methods which can be used to terminate the CmdStan process after the specified time (621).
- A model will now properly recompile if one of the `include`-d files changed since it was last built (627).

- Fixed a critical bug in how outputs containing complex numbers were read in by CmdStanPy. If you are using models which output complex numbers, please update immediately to ensure you are getting the correct results.

- Fixed a build error in the documentation
- Improved messages when model fails to compile due to C++ errors.

- Fixed a typo in `cmdstanpy.show_versions()`
- Reorganized and updated the documentation
- Reorganized a lot of internal code
- Cleaned up the output of `CmdStanMCMC.draws_pd`
- Cleaned up the output of `CmdStanMCMC.summary`
- Removed the logging which occurred when Python exited with cmdstanpy imported.

- Fix an issue with the `cmdstanpy.install_cmdstan()` function where the installation would report that it had failed even when it had not.