[virtual refinement] Renames tags, removes level tag, introduces functions distinguish between intervals and levels

The identity of a reference element is for now based around the notion
that the implementation is a singleton, so comparisons are based on
the stored pointer value.

The old functionality is enabled by default, i.e. using VirtualRefinement functions without setting the VirtualRefinementTag explicitly yields the same behaviour.

Before the number of sub elements along one axis had to be a power of 2, now this restriction is lifted.
Beware that the number of sub elements must be >=1, otherwise the program will crash.

[test][MultiLinearGeometry] Small demonstration of how the reference_wrapped corner storage can be used.

As decided on http://users.dune-project.org/doodles/39

The method never worked, until fixed rather by coincidence right
before the release of dune-geometry-2.4.  So apparently demand
for it was low.

Only for triangles and tetrahedra, though.  That's enough to prove my point.

Similar to autotools, this fixes FS#1548.

This has the effect that the standard library will see the exception and will
usually print information about it, such as the type of the exception.

This worked previously, because we were testing GaussLegendre quadrature rules
first seeding the cache.  Second we were testing GaussLobatto quadrature rules
-- or so we thought: due to FS#1541, the cache would yield the previous
GaussLegendre quadrature rules, even though we requested GaussLobatto.  With
that fixed, the test would try to instantiate GaussLobatte quadrature rules of
up to order 45, even though they are only implemented up to order 31.