[WIP] Feature/new staggered impl

TODO General:

• Fix docu (especially copy and paste errors)
• Rethink assembly strategy (forward / inverse)?
• Introduce coupling stencils per DOF?
• improve design of "dual" problem
• boundary flux helpers
• prohibit Dirichlet for mass model?
• check difference in Jacobian for compressible fluids (channel)
• periodic grids
• Look into benefits of caching options
• Add volume work to energy balance

@nedc:

• Set up higher order geometry
• Port the TVD methods
• Add correct checks for various boundary conditions
• Add useful tests for higher order
• Update and include the rans models

@kweis:

• coupling (staggered-cellcentered)
• implement Beavers-Joseph BC
• Compositional models (1pnc)

@martins

• Finalize box-staggered coupling (old staggered)
• Port box-staggered to new staggered
• Develop new freeflow discretizations (long term)

@hanchuan

• port and test Navier stokes tests (should have been completed already by @kweis)
• port compositional tests (after 1pnc is updated)
• port stokes-darcy MD tests (after MD is updated)
• port rans tests (after rans is updated)

fixes #756 (closed)