find gurobi if present, test on ci

[tlambert03]

this PR:

1. adds the gurobi_c.h header, allowing us to build against gurobi if the library is present (installation of that is still, at this point, left to the user)
2. moves the logic from config.h into setup.py: we no longer hard-code HAVE_GUROBI, it will be determined at buildtime
3. sets up CI to build the package and run tests
4. xfails the gurobi test if no license is present
5. removes requirements.txt files and puts them into setup.py dev extra (use pip install -e .[dev])

[tlambert03]

i'm going to optimistically merge this one to get the CI to run, it doesn't touch the cor library (and the new tests won't run on CI unless it's in master)