add command line args parsing
add missing text to prologue
add functions for parsing model string

Parse model string, as passed to mpi_makemap via command line argument.
This code previously sat in mm_get_cl_args.c, but will have mpi_makemap.c
call these functions instead.
add model type name to struct
update cl_args to store model string instead of parsed model names
Return pointer to name and length rather than copying string
Update mpi_makemap to use mm_model_str functions

Can now specify which models (and order) to run from command line. Also
requires switching to generic interface for calling models, rather than
calling each by name.