New and reorganized documentation
Covers more mdrun options, moves a bit of "practical" content from the
reference manual to the user guide.
Imported and updated information from wiki page on cutoff
schemes. Consolidated with information from reference manual.
Updated some use of "atom" to "particle" in both guides.
We could update the performance numbers, but with the impending
removal of the group scheme, I don't think that's worth bothering
about. e.g. on Haswell, Erik already tested performance of group is a
bit slower than Verlet, even for unbuffered water systems.
Change-Id: I6410ba9fc08bb133ec8669e14dba11bcbd454fe3