WIP: Separate cmake files instead of one big DuneMacros.cmake

Summary

This is a cleanup MR. All cmake functions and macros are now prefixed with dune_. The most important changes are:

• add_dune_*_flags --> dune_add_*_flags
• finalize_dune_project --> dune_finalize_project

The old functions still exist, but are deprecated.

These changes are made to have for all function a common name-base. This allows to tell from the function name whether it is dune-specific or a cmake function or comes from somewhere else.

Further improvements

1. Private macros not part of the public cmake API are separated by a comment "internal macros/functions"
2. Functions are put into separate files with a corresponding name. This would reflect the c++ style guide just for cmake, e.g.

DuneProject.cmake:
- dune_project()
- dune_finalize_project()

DuneAddLibrary.cmake
- dune_add_library()
- some "private" macros used by dune_add_library

DuneHeadercheck.cmake:
- dune_setup_headercheck()
- dune_finalize_headercheck()
- some "private" macros used by headercheck macros