Comments out extern template class statements

The commit comments out the following two statements
   extern template class VertexDoFFunction< double >;
   extern template class EdgeDoFFunction< double >;
in the corresponding header files of the two classes. For some
reason that is currently totally obscure to me I get errors of
the following form when the two statements are active and the
cpp files of the classes are compiled:

VertexDoFFunction.cpp:1765:95: error: specialization of ‘void hyteg::vertexdof::VertexDoFFunction<VType>::invertElementwise(walberla::uint_t, hyteg::DoFType) const [with ValueType = double; walberla::uint_t = long unsigned int]’ after instantiation
 void VertexDoFFunction< real_t >::invertElementwise( const uint_t level, const DoFType flag ) const

This happens with GCC 8.3. The error seems strange to me as my
understanding of the statements is that they suppress instantiation.
24 jobs for mohr/ElementwiseOperators-mergeCandidate in 56 minutes and 58 seconds (queued for 3 seconds)
Name Stage Failure
clang_8.0_eigen_petsc Pretest
[  8%] Linking CXX static library libsqlite.a
[ 8%] Built target sqlite
[ 8%] Linking CXX static library libcore.a
[ 8%] Built target core
make[1]: *** [src/hyteg/CMakeFiles/hyteg.dir/all] Error 2
CMakeFiles/Makefile2:15397: recipe for target 'src/hyteg/CMakeFiles/hyteg.dir/all' failed
make: *** [all] Error 2
Makefile:140: recipe for target 'all' failed
ERROR: Job failed: exit code 1