[examples] Fix matrix/vector types of examples

The nested matrix and vector types should correspond to the index tree of the basis. Here, this is not the case since Field*<T,*> was used instead of T. While this is in many cases no problem, it may fail if the matrix is recursively traversed with IsNumber as termination criterion.

While it is not obvious, this is the first of a sequence of changes towards getting rid of the old basis implementations.

• Before removing the old bases we should have hanging node support for the new ones.
• With hanging nodes the old utility for incorporating Dirichlet values no longer works and has to be replaced by a generic matrix traversal algorithm.
• Generic matrix traversal requires the matrix nesting structure and multi-index type match.

enabled an automatic merge when all merge checks for 5e9faaed pass

mentioned in commit fe543f9c

merged