Improve visibility of documentation and tests

[gareth-d-ga]

At the moment:

• rptha has documentation in R format, extracted from Roxygen-style comments in the code.
• other R scripts often use Roxygen-style comments in the code, although as they are not in packages, they are not processed into browse-able formats.
• SWALS has source-code documentation which can be converted to html via Ford.
• All the SWALS validation tests can be run and produce figures.

It would be nice to make all this more easily accessible via the web.

[gareth-d-ga]

README.md blurbs are being added to the SWALS validation tests, and it is possible to merge them into a pdf

[gareth-d-ga]

We've now got the SWALS validation test README.md files to link to figures (stored on another branch to avoid repo bloat). This makes it much easier to see the SWALS tests.

• See subfolders here for general tests: https://github.com/GeoscienceAustralia/ptha/tree/master/propagation/SWALS/examples
• See subfolders here for nthmp tests: https://github.com/GeoscienceAustralia/ptha/tree/master/propagation/SWALS/examples/nthmp