Change diffusion -> viscosity.
Remove quadratic_friction option - now is always True (removes linear drag support).
Rename diffusion -> viscosity in tests and examples as well.
viscosity instead of viscosity_coef
Remove newton_solver param.

Since we've remove linear friction we now always have a non-linear solve which
we now always solve using Newton (removing Picard support).
Remove all mentioning of Picard.
Remove quadratic_friction option from tests.
Remove linear friction version o fthis test.
No longer running linear friction case.
Small website update
Merge branch 'cleanup-swe-solver' of github.com:OpenTidalFarm/OpenTidalFarm into cleanup-swe-solver
Add a much faster version of the mms_flather test
Some more tweaks to the mms_flather test
Slightly tidy up the constraint-derivative test
Start using the new logging functions
Speed up the dynamic control test
Some first speed ups for gradient_configurations
gradient_configuration takes below 10s now
Speed up the weak Dirichlet boundary tests
Speed up mms_strong_dirichlet test