Add option to provide PROCESS solver by external package

[timothy-nunn]

Description

Allows PROCESS' --solver option to try and import the solver from a package iff the provided string is not an inbuilt PROCESS solver. The string must be a valid (sub)package that provides a __process_solver__ attributes which points to a class that subclasses _Solver.

Checklist

I confirm that I have completed the following checks:

• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[jonmaddock]

Fantastic Tim, great solution. Can you get some documentation sorted for this too (when you let the group know)?