add examples in geocubit

adding some example on creating the mesh using geocubit.

homogeneous 1 layer/2 layers/tripling
1 tomographic
2 topographic mesh
3 water layer
1 pre-existing mesh
Merge pull request #151 from casarotti/devel

add examples in geocubit
add irregular topography option...
delete block_mesh.py
bug fix

use list instead of integer for mpi.allgather()
Merge pull request #152 from casarotti/devel

bug fix and add irregular grid option in geocubit