Add an option to build SWIG python bindings with threads enabled

[kkg4theweb]

MEEP is currently built with threads disabled by default for the SWIG python bindings. This means all calls swigged C++ code hold the GIL by default. This has the unfortunate side-effect of long running MEEP calls holding up the python interpreter and not being able to do anything else in parallel even if it is safe to do so.

We have a few options

1. Do nothing
2. Release the GIL on selected calls that we know to be long-running
3. Enable 'threads' (optionally) for ALL of SWIG python bindings

[smartalecH]

Similar discussion here.

[smartalecH]

Does #1762 resolve this (very nice work btw)?

Did you by chance run any experiments to see if this works well (e.g. running on 1 proc with multiple threads and comparing speedup to before)? I see it's been added to CI