[tests.linearelliptic] introduce EstimatorStudyBase and derive EocStudyBase
[tests.linearelliptic-swipdg] update
Revert "[tests.linearelliptic] introduce EstimatorStudyBase and derive EocStudyBase"

This reverts commit e48fa290d4cb6e0b407d87923db497f53e060257.
[linearelliptic.testcases.base] fixed assertion
[tests.linearelliptic] fixed assertions
[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.
[tests.linearelliptic-swipdg] update to provide norms and estimators
[linearelliptic.discretizations.swipdg] added some estimators