[bugfix]Various fixes for DataCollector implementations

This fixes various bugs in DataCollector making them unusable for certain GeometryType combinations:

• Fix DiscontinuousDataCollector for permuted GeometryTypes. There was one permutation missing in DiscontinuousDataCollector. Hence the elements where distorted if the permutation is not trivial, e.g. for quads and hexahedra.
• Fix DiscontinuousLagrangeDataCollector for mixed grids. Indices in the IndexSets of a grid are only unique wrt. the GeometryType. Hence one has to compute per GeometryType offsets for mixed grids. Here, these offsets will depend on the size of the local point set. This can easily be done by replacing the IndexSet by a mapper using a layout which associated blocks of appropriate size to each cell GeometryType.
• Fix LagrangeDataCollector for mixed grids analogously to DiscontinuousLagrangeDataCollector.

Fixes #8 (closed).