Move find_package(PTScotch) to DuneCommonMacros
In dune-common a
FindPTScotch module is provided. This MR moves the
DuneCommonMacros.cmake, so that you actually get this package registered into the dune-common module.
find_package was called inside other find module. This leads to the message "PTScotch found", but you cannot use it if not explicitly including the
AddPTScotchFlags cmake file.
Closes #250 (closed)