[test.OS2014] fixtures seem to be overkill
[test.linearelliptic...] remove trailing _ for private methods
[linearelliptic.problems.interfaces] added NonparametricType for with_mu()
[linearelliptic.discretizations.swipdg] restructured estimator

The estimator now takes a space, a vector and a problem (instead of the
discretization and a vector) and may thus be used by other discretizations.
[test.linearelliptic-swipdg] use estimator directly
put some stuff into .cc s
[cmake] update