[linearelliptic.discretizations.cg] clean up
[linearelliptic.problems] added type
[linearelliptic.problems.interfaces] added report() and operator<<
[linearelliptic.discreteproblem]
* updated config tree usage
* added visualize flag
* added problem report
[linearelliptic.problems.ESV2007]
* use correct functions
* fix dirichlet boundary values
[linearelliptic.problems.mixed-boundaries] added first draft
[linearelliptic.problems] refactor ProblemProvider
[linearelliptic.discretizations.base] updated exceptions
[linearelliptic.discretizations.cg]
* update space and la choosing
[linearelliptic.discretizations.swipdg-...] removed
[linearelliptic.discretizations.swipdg] added first draft to playground
[examples.linearelliptic.cg] updated
[examples.linearelliptic.swipdg] added first draft
[cmake] added examples.linearelliptic.swipdg