Tidy comments/docs on GMake API functions
Add function for checking dataset existence

Provide g4gmake_check_dataset(s) function(s) for checking that a named
dataset variable's value  points to an existing directory. Take dataset
name as argument to singular function form, erroring out if dataset name
is unknown or directory exists. In plural function, simply loop over all
dataset names stored in G4GMAKE_DATASET_NAMES.