Refactoring assembly of RichardsMechanics

This is the first step in an endeavour to restructure the assembly of the RichardsMechanics process in a fashion similar to TRM/TH2M/SD/LD.

This MR

• adds some (mostly) empty/boilerplate files for constitutive models/data/...
• moves some of the code of assembleWithJacobian() to a separate method.
• note: RM supports staggered coupling. This MR only changes the monolithic assembleWithJacobian().
• no behaviour change intended.

Review of the changes in the local assembler might be easier with git's moved code detection:

git diff origin/master --color-moved --color-moved-ws=ignore-all-space -- ProcessLib/RichardsMechanics/RichardsMechanicsFEM-impl.h