[linearelliptic...swipdg] updated estimator
[linearelliptic...swipdg] updated estimator

Moved ComputeEstimator out of the discretization and replaced local
nonconformity estimator by an appropriate functor.

tobiasleibner/dune-hdd,tobiasleibner/dune-hdd,pymor/dune-hdd,pymor/dune-hdd