rename "(Volume|Flux)Variables" to "(In|Ex)tensiveQuantities"

"intensive" means that the value of these quantities at a given
spatial location does not depend on any value of the neighboring
intensive quantities. In contrast, "extensive" quantities depend in
the intensive quantities of the environment of the spatial location.

this change is necessary is because the previous nomenclature was very
specific to finite volume discretizations, but the models themselves
were already rather generic. (i.e., "volume variables" are the
intensive quantities of finite volume methods and "flux variables"
are the extensive ones.)
replace 'dune_static_assert' by 'static_assert'

the oldest target compiler (GCC 4.4) supports this...