CBM experiment using sis18

An interest in running simulations of the CBM experiment with beam from sis18. Adds much setup
based on full electron setup. Magnet, TRD, and psd geometries removed from setups.
Magnetic field same as before but rescaled to 30%.

GDML script for beam window, effectively a smaller STS beampipe is included.
GDML script for elarged target (aka MVD) box design.
pipe_v22c which is root geometry binary with combines these two geometries.