add decimate flag

using          GEOCUBIT.py --collect   --decimate --merge --meshfiles=[list of files] --cpux=N --cpuy=N (--rangecpux=[cpuxmin,cpuxmax], --rangecpuy=[cpuymin,cpuymax])
it divide a hex in 8 piece doubling the resolution (but keeping the coarse representation of the interfaces like the topography)

!!!! we need to test this feature