Upstream FieldTraits specialization and possibly propose default typedef changes

https://git.imp.fu-berlin.de/agnumpde/dune-matrix-vector/blob/master/dune/matrix-vector/traits/fieldtraits.hh (which currently only specializes FieldTraits for ScaledIdentityMatrix) should be upstreamed to dune-istl, since it requires special knowledge that this file should be included to get the expected field_t- and FieldTraits-behaviour.

Unfortunately, the default typedefs in FieldTraits (dune-common:https://gitlab.dune-project.org/core/dune-common/blob/master/dune/common/ftraits.hh) are also chosen awkwardly IMHO, as they serve as a recursion anchor for nested types, but do not provide insightful compiling errors when used on unspecialized types (like above). I'd rather see some enable_if<is_arithmetic<T>>-like recursion anchor there as well.