Use static assertion to ensure a coefficient deduction in HierarchicVectorWrapper

If HierarchicVectorView is not passed the coefficient type directly, it will try to deduce it which fails for dynamically sized multi-indices. This is e.g. the case for non flat indices if you do not create the wrapper manually.

This problem is now captured by a static_assert. In order to see this in the error message the default coefficient type used to indicate that it should be deduced had to be changed from void to a named type. Otherwise you get errors for code using the result before even seeing the failed assertion error. Using a named type here is a good idea anyway.