[Whitespace] Clean up whitespace in gridoperator/common/CMakeLists.txt
[Bugfix][CMake] Distribute all files in gridoperator/common
[Bugfix][Buildsystem] Distribute diagonallocamatrix.hh in gridoperator/common

Looks like I forgot to add that file to both build systems. Fixed.
[Whitespace] Clean up whitespace in gridoperator/common/CMakeLists.txt
[Bugfix][Buildsystems] Install pkqkfem.hh and qkdggl.hh
[Bugfix][CMake] Add missing header files to CMakeLists.txt
[Bugfix][PkFEM] Make sure index compression for 2D FE variant selection is consistent

The PkFEM in 2D uses an array of 6 different FE variants for the
different possible permutations of the vertex indices. When looking up a
finite element, the subindices of the vertices are retrieved and
compressed to the range [0,5] depending on their permutation state.

Unfortunately, the code doing uses the inverse of the correct statement
for the relative ordering for vertices 1 and 2.

Fixed - thanks to Pavel for noticing!
Merge branch 'releases/2.0'

Bring in a bunch of additional bugfixes from the release branch

* releases/2.0:
  [Bugfix][PkFEM] Make sure index compression for 2D FE variant selection is consistent
  [Bugfix][CMake] Add missing header files to CMakeLists.txt
  [Bugfix][Buildsystems] Install pkqkfem.hh and qkdggl.hh
  [Whitespace] Clean up whitespace in gridoperator/common/CMakeLists.txt
  [Bugfix][Buildsystem] Distribute diagonallocamatrix.hh in gridoperator/common
  [Bugfix][CMake] Distribute all files in gridoperator/common
  [Whitespace] Clean up whitespace in gridoperator/common/CMakeLists.txt