Clarify install instructions for ML processors that are NOT python packages

[tjd2002]

In several places the install instructions for ML processor packages presume that these are well-formed python packages (1) git clone; (2)pip install; (3)ml-link-python-module).

This led to confusion in our lab, when users tried to use the ml-link-python-module command to link a ML processor package (which was just a folder containing python scripts, not a proper python package). Instead, they needed to manually create symlinks from the processor package search path to their ML processor package directories, but this step is not documented.

[tjd2002]

cc @mari-sosa

[magland]

I think the developer installation instructions are technically correct as they stand: https://github.com/flatironinstitute/mountainsort_examples

Note the step: export PYTHONPATH=[fill-in-path]/ml_ms4alg:$PYTHONPATH

After doing that, ml-link-python-module should work, and does the same thing as manually creating the symbolic link.

However, I agree this is confusing. Should we change these docs to a ln -s ... command? Or are there other places where this should be clarified? @tjd2002