[tests.linearelliptic] extend EocStudyBase to norms and estimators
[tests.linearelliptic] extend EocStudyBase to norms and estimators

Any derived class can implement available_norms_() which will lead to
compute_norm_() being called and/or available_estimators_() which will
lead to estimate_() being called.

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