assembler member function.

Rename namespaces and classes.

Rename files and change includes.
Renamed:
- GroundwaterFlowProcess.{h,cpp} to SteadyStateDiffusion.{h,cpp}.
- GroundwaterFlowProcessData.h to SteadyStateDiffusion.h.
- GroundwaterFlowFEM.h to SteadyStateDiffusionFEM.h.

- Rename
  + process name,
  + process folder, and
  + cmake macros.

Passing a vector of solutions now.

Fixes multiple copyright messages in the doxygen docu.

ag '\\file' -L --ignore=ThirdParty *Lib |xargs  sed -i 's/\(.*\)\\copyright/\1\\file\n\1\\copyright/'

std::vector<std::size_t> _ids_of_active_elements.

for flexibility in coupling.

 VectorMatrixAssemble::assembleWithJacobian

Clang format run on near-the-header lines.

Happy New Year

pointer to StaggeredCouplingTerm

 coupled processes.

 for the staggered scheme.

coupled terms.