mark 'write as model' as complete
each process reads chunk of detectors
remove '#ifdef USE_MPI' clause -- is defined in config.h
rename nprocs to nproc
add rank/nproc to exec/calc argument lists
add mpi to Makefiles, model header
change nmappix from size_t to int
add MPI communications for model_ast_calc

Ast model now correctly calculates map by splitting data between processes,
each calculated map on chunk, and using MPI to combine the separate maps.
mark 'write communications for ast mode' as done