XXX this function will be augmented and possibly superceded by a new QM-capable version named read_timestep(), when finished.
Read the next timestep from the file. Return MOLFILE_SUCCESS, or MOLFILE_EOF on EOF. If the molfile_timestep_t argument is NULL, then the frame should be skipped. Otherwise, the application must prepare molfile_timestep_t by allocating space in coords for the corresponding number of coordinates. The natoms parameter exists because some coordinate file formats (like CRD) cannot determine for themselves how many atoms are in a timestep; the app must therefore obtain this information elsewhere and provide it to the plugin.