added angle-parameter to extract surfaces more precisely
formatting
find x
changes based on PR comments
removed binary file and fixed intentation
exchanged pi-constant for radian-constant
added doxygen filename tags
Reduce indentation by reordering if clause.
Use std::size_t instead of size_t.
getSurfaceAreaForNodes() returns its result.

Also correct a description.
manually added changes by @endJunction that couldn't be added due to merge conflicts
replaced boost::radian with boost::pi again because the former didn't give the desired result
transferred OGSFileConverter-files from OGS5 to OGS6 and adjusted everything as needed
adjusted cmake files
fixed logog-problem and optimised code a little
added license information
replaced external call to file converter with instance of transferred dlg
removed file converter path from preferences dlg
Removed unused Header variables.

This speeds up configuration especially on Windows.
Merge pull request #495 from bilke/cmake-cleanup

Removed unused Header CMake variables.