Move find_package(PTScotch) to DuneCommonMacros
Summary
In dune-common a FindPTScotch module is provided. This MR moves the find_package(PTScotch) and include(AddPTScotchFlags) to DuneCommonMacros.cmake, so that you actually get this package registered into the dune-common module.
Details
Before, the 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)
Edited by Simon Praetorius