Remove the `pip install` lines in `Makefile` and add the `--no-deps` flag for the local install

[drbenvincent]

@lucianopaz pointed out that having pip install commands in the Makefile could potentially lead to version conflicts that can't be resolved. So we should perhaps remove these lines in the Makefile.

Add the --no-deps flag when doing the local import of the CausalPy package. The purpose of this is to ensure that we have a clean condo-only install of pymc.

Double check the CONTRIBUTING.md file to ensure instructions are up to date.