Automatic/parametric finding of external solvers

Created by: acrovato

Context

The external solver are currently "linked" to CUpyDO by setting their install/build directory in the PYTHONPATH.

Issue

It is not convenient/easy for users and might be annoying for developers who would want to run on a stable version of the solver and develop/test on a development version of the same solver.

Proposed solution

We could use a "run.py" system (suggested by @rboman) or a configuration script (suggested by @lpapeleux)