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
Began a section in the documentation with guidelines for contributors.
Hopefully fix some of my dreadful reST...
More reST fixes...
Yet more fixes...
Fixed some inline code examples
Fix typo.
Started adding language rules and guidelines
Added a small section about logging.

* Instructions to use the logger.
* Instructions to use the progress bar in computationally expensive cases.
Add a much faster version of the mms_flather test