add default initialization of reserved vector

Simon Praetoriusrequested to merge
simon.praetorius/dune-common:issue/non_initialized_reservedvector into master

Summary

Fixes a warning of maybe not initialized member of ReservedVector

Details

A default initialization of a ReservedVector, i.e.

ReservedVector<double,3> vec;
vec.push_back(1.0);
vec.back() = 2.0;

might result in a compiler warning about possible use of uninitialized members vec.data[0]. This can be fixed with a default constructor and default member initializers.

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