PPA for ghc-8.10.7 is not available

Improve CI:

* Use new version of git-modtime

* Fixed caching on MacOS and Windows:
  * Disable caching of stack-work on MacOS, problems running cached binaries.
  * Add caching stack-root on Windows

* Update to newer version of shc

Update ghc versions package is tested against

Fixups to haddock

Fixup haddock

Fixup some haddock

Revert some breaking changes from 5423357

Fix readme and lower bound of cabal

Addition of Wikipedia 2 degree illuminants. Addition of `convertColor…

…` and `convertColorFloat`. More tests and doc. A very good comparison to colour package

Coveralls isn't too happy with symbolic links in README