create submodels.h header file
add variable for number of map pixels

Also, use malloc instead of calloc for map initializations -- maps will be
zeroed on each call to model_ast_calc.
add varmap and hitsmap to model_ast data structure
rename argument
specify interface for model_calc_func function pointer
add exec_model wrapper function
do model_ast calculations -- not yet tested for correctness
remove debugging print statements
mpi_makemap now reads data and processes ast model

Still haven't tested ast_model_calc code!
add a bit more description to header