cleanup boundary conditions

Praetorius, Simonrequested to merge
feature/boundary_condition_constructor into master

This MR attempts to simplify the boundary condition interface by providing:

  • an abstract interface
  • more documentation
  • clearer function names
  • deduction for some template arguments

The introduction of virtual functions requires additional template arguments Mat, Sol, Rhs. Those may be removed later with the change to local application of constraints.

Edited by Müller, Felix

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