Add a specialization of FieldTraits for VariableBlockVector

Simon Praetoriusrequested to merge
issue/missing-fieldtraits-vbvector into master

Summary

This VariableBlockVector has a type field_type but not real_type, it also does not specialize the FieldTraits type-traits class from dune-common. This makes it unusable within a MultiTypeBlockVector, for example. This MR adds the FieldTraits specialization.

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