Implemented g-h-k filter, and extensive documentation.

Still ongoing, but I have added a lot of documentation in the form
of comments to the gh filter.

Implemented some basic tests for the g-h filter (none for g-h-k yet).
Added test using g-h filter form of Least Squares

Least Squares order 1 filter can be implemented with a g-h filter.
I test the two alternative forms of the filter to ensure both give
identical results.
Slight formatting changes