Implement support for manual index transformations

Add TransformedIndexPreBasis and demonstrate its usage.

This pre-basis transforms the global multi-indices of a wrapped pre-basis. This functionality can be helpful to create indexing schemes which cannot be constructed using the index merging strategies of PowerPreBasis and CompositePreBasis.

While the implementation works as desired. This is marked WIP, because I'm neither convinced by the design of interface and implementation, nor is this suitably tested and documented.

For an example have a look at patch for stokes-taylorhood.cc which is included. This transforms one indexing scheme for the Taylor-Hood basis into another. To keep the example program as is otherwise, the target indexing scheme coincides with the one in the unpatched example.