Merge branch 'bug/parametertree_complex' into 'master'

ref:core/dune-common Older compilers seem to have problems with std::complex
and \>\>. This leads to the problem that "1" cannot be interpret as a
std::complex with the ParameterTree. See: [https://gcc.godbolt.org/z/5G-we3]

We change the type of dummy to char to avoid this problems.

See merge request [!726]

  [https://gcc.godbolt.org/z/5G-we3]: https://gcc.godbolt.org/z/5G-we3
  [!726]: gitlab.dune-project.org/core/dune-common/merge_requests/726