[mapper.finitevolume] added first draft
* based on the indexSet() of the underlying grid view
* seems to work
[basefunctionset.finitevolume] added first draft
* untested
[space.finitevolume] added first draft
* seems to work
[operator.projections] added L2 projection for FV space
[space.cg.fem] removed obsolete continuous()
[space.dg] whitespace
[product.ESV2007] added WeightedL2
* seems to work
* not yet localizable/assemblable
[tests.elliptic-swipdg...] updated residual estimator (ESV2007 variant)
* seems to work, results correspond to the published ones