Cleanup/apply naming scheme to tests

General Plan:

1. Rename targets.
2. Reorganize such that only one .cc file exists per folder, rename to main.cc, problem.hh, spatialparams.hh and params.input. Differentiate by adding _xxx if necessary, e.g. problem_darcy.hh and problem_stokes.hh in a multidomain test.
3. Rename Problem.Name to target name and reference solutions to Problem.Name-reference.vt*.

Felix:

• 1p
• 1pnc
• 1pncmin
• richards
• richardsnc

Simon:

• 2p
• 2p1c
• 2p2c
• 2pnc
• 2pncmin
• 3p
• 3p3c
• 3pwateroil

Johannes:

• mpnc
• co2
• tracer
• geomechanics
• discretization (Reverted)

Ned:

• freeflow
• md_boundary
• md_embedded
• md_poromechanics
• md_facet