Allow snopt module to be loaded non-relatively

[whophil]

Description of feature

Currently the snopt.*.so binding module is loaded relatively to pySNOPT.py - it is expected to live in the same folder.

pyoptsparse/pyoptsparse/pySNOPT/pySNOPT.py

Line 7 in 40af7dc

from . import snopt # isort: skip

This is a problem when installing pyoptsparse from conda-forge, because the snopt module is not included in the conda-forge release, due to license restrictions.

Consider the case where a user wishes to:

1. Install pyoptsparse from conda-forge
2. Provide their own pre-compiled snopt.*.so binding module

As currently implemented, the user would have to copy it into the environment's site-packages folder to allow snopt to be imported correctly. This is not desirable.

Potential solution

An environment variable which allows the snopt module to be loaded non-relatively should do the trick, as a user can put whatever path onto their module search path with PYTHONPATH.

[whophil]

Closed by #338