This merge request adds a multidomain module to Dumux. It is a fairly generic module that provides a generic assembler for multi-domain problems (more than two domains possible). The domains can have different dimension. The coupling manager concept allows to specify coupling dof dependencies and defines how to evaluate coupling residuals / residual derivatives.

The goal is to be able to use this module for

• equal-dimension multi-domain problems (e.g. Darcy-Stokes coupling)
• mixed-dimension multi-domain problems (e.g. embedded mixed-dimension methods, embedded fracture models)
• multi-physics problems (e.g. dual-continuum models, geomechanics-flow)

New features / models on this branch

• Staggered grid (is using multidomain now, multi-physics: facet and cell-center domain)
• Embedded coupling (mixeddimension non-conforming) (embedded tubes and fractures)
• Facet coupling (mixeddimension conforming) (fractures and 3d-2d-1d)
• Geomechanics (Biot/el1p, multi-physics)
• Boundary coupling (equaldimension, darcy-stokes-> !997 (merged), darcy-darcy-> !1019 (merged))
• ( not on this branch but bare in mind: boundary coupling, mixeddimension porenetwork-darcy/stokes)

Plan (TODOs)

• Squash and reorder commits into sensible compiling patches
• Fix documentation
• Fix CMakeLists.txt (done in b3098aa6, there is no a script for that on master)
• Add reference solutions for multidomain tests
• depends on !737 (merged) and !738 (merged) to be merged.
• check function overloads for different element types, probably need domainId as element types can be the same
• add coupling manager for darcy-darcy domain decomposition (show case / test and simpler than darcy-stokes) (see !1019 (merged))
• add darcy-stokes module (see !997 (merged))
• Unify newtoncontroller with staggered newtoncontroller (depends on !762 (merged))
• Make facet grid creator a grid manager with grid data management
• Find better name for EmbedmentData in facet coupling grid manader

Will be a separate merge request

• implement explicit assembly (see #521 (closed))
• Sequential solver (see !870)

Fixes #447 (closed).