Cleanup MultiIndex generation and use

In order to support hybrid (i.e. runtime-sized) multi-indices, the PowerPreBasis and CompositePrebasis currently rely on using ReservedVector as MultiIndex. The allocated size is determined automatically using *PreBasisFactory::requiredMultiIndexSize and the determined MultiIndex type is forwarded to all involved PreBasis instances as template parameter. This has a severe drawback: For uniform (i.e. with statically known size) multi-indices, you cannot use a statically sized MultiIndex like std::array. The reason for this is that PowerPreBasis and CompositePrebasis need to know the size of the multi-indices generated by its children while constructing the final exported multi-indices. Storing this size the way ReservedVector does it is most natural.

As a remedy I propose the following changes:

• Rename *PreBasisFactory::requiredMultiIndexSize to *PreBasisFactory::maxMultiIndexSize and add *PreBasisFactory::minMultiIndexSize. This allows to detect uniform multi-indices.
• Allow different types for stored and exported multi-indices. Using ReservedVector for caching allows to build multi-indices as outlined above. For uniform multi-indices the statically sized internal array is better suited.

In order to implement this we have to:

• Remove/ignore MultiIndex template parameter from *PreBasis and *NodeIndexSet. The type of the stored multi-index can be deduced in the indices() method and the type of the exportet multi-indices is not needed.
• Add template parameters CachedMultiIndex and MultiIndex to DefaultNodalBasis and DefaultLocalIndexSet and let them decay multi-indices from the cached to the exported type automatically.
• As an alternative: Pass minMultiIndexSize and maxMultiIndexSize to DefaultNodalBasis and hard-wire appropriate CachedMultiIndex and MultiIndex types.