Add pydrex.viscosity module

[adigitoleo]

First part of adding variable viscosity support. The main things I add here are:

• pydrex.viscosity module with functions to get effective viscosity contributions from different mechanisms
• a bunch of references and details to docstrings of the default parameter choices
• a draft of the Fluidity 2D ridge variable viscosity example, but without the actual variable viscosity implemented yet (will add that in a part 2, this flml is just from an older branch)
• support for custom constraints in the gmsh rectangular mesh generator (so I can have high resolution at the top centre, for 2D ridges)

Unfortunately the ridge example doesn't work with the mesh generator for some reason, so I have to use a .geo file instead. Might be able to fix this at some point but will focus on getting the full variable viscosity tests validated first.

[Patol75]

Looking great so far; another significant contribution!