Adjusted relative paths of CMakeLists.txt

[JanNiklasB]

Hi,

as mentioned in issue #523 when including CRPropa over add_subdirectory in other projects, the relative paths are not pointing to the correct folders.

To better support CRPropas behavior when included elsewhere, I changed all relative paths to the correct absolute paths utilizing CMAKE_CURRENT_SOURCE_DIR and CMAKE_CURRENT_BINARY_DIR respectively.

I also added a new cache variable for CRPROPA_EXTRA_INCLUDES so the include paths can be used on the above levels (I do not know how to hide this from the user if possible).

[JanNiklasB]

Here is a short example where the problem might occur when using the CRPropa:master instead of JanNiklasB:master:

https://gist.github.com/JanNiklasB/ca187681f446d323bdc773c425d2e57f

The issue is that when CRPropa is compiled as subproject, the main project can not find the includes of the crpropa subproject (like for example KISS), since those are first added to the usual include folder in the install step which is never done during FetchContent.
The solution is to add all CRPROPA_EXTRA_INCLUDES as hidden cache variable (with mark_as_advanced) so the main project can include those as well.

[JanNiklasB]

I merged the newest master and took the opportunity to make some other small changes.