add new define so that the interface check is not always used as soon as
NDEBUG is undefined (allows to tests asserts without doubling call to all
interface methods).

NDEBUG needs to be undefined and DUNE_INTERFACECHECK needs to be defined
to check the correct implementation of CRT interface methods.
[Densematrix] Explicitly cast '0' to field type to support user-defined field types
Merge remote-tracking branch 'upstream/master' into threads