[linearelliptic.discretizations.swipdg] added estimator
[linearelliptic.discretizations.swipdg]

Move the computation of the estimator into its own ComputeEstimator class,
specialized by the GridType and the dimensions. This allows us to compile
the discretization without estimator for arbitrary grids and dimensions.
[linearelliptic.discretizations.swipdg]

Add compile flag to carry out a different kind of summation of the local
estimator contributions. This is the way it's done in the OS20013
publication and yields a slightly less efficient estimator.