MoL multirate lacks documentation

[Roland Haas]

#839 did not provide updates to MoL's documentation so that the only way to learn how to use it is to reverse engineer what GRHydro does.

Keyword: MoL
Keyword: Multirate

[Roland Haas]

Added documentation for multirate scheme. Text is mostly cropped from Christian Reisswig's GRHydroMP paper (with permission).

Added some more text to describe functions and parameters.

[Frank Löffler]

Please apply. Documentation usually doesn't need a review either, here that is. :)

[Roland Haas]

Documentation was added in rev 184 if MoL.