adds capability of using negative material ids in xmeshfem3D for tomographic models
fixes compilation error on cray for mpi status in read_sep_binary_mpiio; removes trailing spaces and warning about unused function
Adds a tool to create ascii file for bathymetry from sep files
Adds readme for the bathy_sep_to_ascii tool
Removed Makefile -- generated by cmake
adds src/tomography directory with summation, smoothing and model update tools
Modif to fix problem of generate databases with DSM