Make python version consistent across boost, py_detector_interface, and simex

[CFGrote]

When building simex on maxwell, the boost lib in /data/netapp/s2e/simex/lib is used, which is compiled against python3.6. python3.6 is then also picked up by py_detector_interface, while simex itself picks up whatever is pointed to by Python_ROOT_DIR. This must become consistent:

Build boost only if XCSITPhotonDetector is requested
Set env so that boost is linked against the same python as simex

ensure the above also for py_detector_interface

There is already a branch (off develop): update_py_xcsit