Document parallel boundary conditions and 'aligned trasform' parallel derivatives

[johnomotani]

Add documentation for parallel boundary conditions. Fixes #2531.

Add documentation for 'aligned transform' variant of shifted metric parallel derivatives (as used by STORM, Hermes). 'Aligned transform' is the version that calculates parallel derivatives by transforming to a globally field aligned grid, calculating the operation, and transforming the result back to the toroidal grid. I've called it 'aligned transform' to get a short name - any suggestions for a better name very welcome!

• [ ] @dschwoerer, @bshanahan, @ZedThree please check the section on FCI bcs - feel free to edit/expand.