Unify constructors to always allow a .stan file to be passed

[WardBrian]

Closes #187.

Changes:

• Python: StanModel.from_stan_file is deprecated. The logic is subsumed in the constructor, which will trigger compilation if the file provided ends in .stan
• Python: The model_data argument is renamed to data. The previous argument still works but issues a warning.
• Python: All functions which accepted strings for file paths now also accept the os.PathLike interface
• Julia: The StanModel(;stan_file,...) overload is deprecated. This logic is now subsumed in the constructor, which will trigger compilation if the file provided ends in .stan

[roualdes]

Overall this looks great. Just a general questions about putting deprecation warnings in doc strings, to compliment error messages.

[roualdes]

Is None better here than ""?

[WardBrian]

None ends up being passed as nullptr to C++, which then creates an empty_var_context:

https://github.com/roualdes/bridgestan/blob/41b66b7956817ecb411e7dccedeed4d6fd8347dc/src/model_rng.cpp#L65C1-L67C56

If you pass "" instead, this string is then treated as an empty JSON, which creates a json parser/var_context and populates it with nothing.

So they're semantically equivalent, but trigger different code paths

[roualdes]

Thanks.