Dynamic library loading
Replaces https://gitlab.cern.ch/simonspa/allpix-squared/merge_requests/8
This basically works. There are currently two main issues:
- Loading is now done with the ALLPIX_DIR variable (or the local directory if it does not exist). This has to be fixed to use RPATH instead (and only after this possibly the ALLPIX_DIR or a config variable).
- Geant4 is compiled with the initial-exec TLS model (thread local storage model). This is giving problems when too large thread storage is loaded. Possible workaround is to use LD_PRELOAD, but we have to look at a better solution if possible (or otherwise instruct the user how to handle it)...
Furthermore some testing has to be done to check if everything works fine before we merge this.