Handle optional dependencies (Par)METIS and (PT)Scotch by directly passing HAVE_xxx flags

Simon Praetoriusrequested to merge
feature/have-metis into master

Summary

This MR changes the way optional dependencies are enabled in cmake, following the MR !1294 (merged). Essentially the add_dune_xy_flags function and the dune_register_package_flags now sets HAVE_XY directory instead of writing a config.h entry and setting ENABLE_XY afterwards.

In this MR the package flags for METIS, ParMETIS, and (PT)Scotch are updated.

Edited by Simon Praetorius

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