CMake refactoring

The Python bindings (pypmt) had some duplicated code to handle which PMT backends should be included or not. This is solved by letting pypmt.cpp include the installed pmt.h header file. However, since the source directory was set globally, it would pick up the internal pmt.h file. To fix this, the global include_directories directive in the top-level CMakeLists.txt has been removed and more specific target_include_directories directives are added wherever needed. The CMake code to replicate the 'installed' PMT headers in the build directory is now always included, such that they are found when pypmt is build.