Add macOS build

[jiuguangw]

This PR adds a macOS CI build.

The current Ubuntu build is not particularly meaningful because it does not install external dependencies and skips several tests. I addressed this by:

• Downloading the pomdp-solve macOS binary. I attempted to build it from source but was unsuccessful (in both Ubuntu and macOS). I did not try the Ubuntu binary.
• Building SARSOP (specifically, the fork for macOS) from source. Binaries are not supplied, and I had to make some modifications to the source code in order to build it successfully (the changes are saved as tests/appl.patch). I couldn't build it in Ubuntu (but didn't try very hard)
• Exported POMDP_SOLVE_PATH, POMDPCONVERT_PATH, and POMDP_SOLVE_PATH. I think it's useful for these to exist in script form.
• Added a CI check for pre-commit

Right now all 7 unit tests pass under macOS. I believe these are important features to facilitate future contributions to pomdp-py.

[zkytony]

Thank you so much! Agree - auto-setup of external solvers should be on the todo list.