fix problem of stopping tracking of particles by geant4 in the middle of the detector

problem caused by a too small top volume, units in cave.geo are mm ! and not cm as everywhere.
This file should be retired in the future and all geometry should be defined using python with
proper units.
first implementation of a geometry framework.
basic parameters are defined in python/ShipGeo.py and
can be imported to any other python module/script.
They are used in python/shipDet_conf.py to configure the
C++ implementation of the Ship detector objects.