Add environment variable for output directory

[tdrwenski]

This PR:

• Adds environment variable QUANDARY_BASE_DATADIR to quandary.py to set a base output directory for both the config.cfg and output files. A user can still pass in a datadir and (if this is a relative path) this will be evaluated relative to the QUANDARY_BASE_DATADIR or if that is unset, then it will use the current working directory. When the environment variable is unset the behavior should be the same as it is currently. As the python code changes directories before executing quandary, it was not necessary to make changes to the C++ code for this.
• Adds tests for this to tests/python. These are also regression tests but need to be in their own folder, since the other regression tests rely on the folder structure for test cases.
• Add a regression and performance test category to pytest to offer an extra way to run the different test subsets in addition to using the folders. pytest -m regression will run both the tests/python and tests/regression. Update docs to explain this.