Renamed grid_molecule struct to surface_molecule.

Also renamed a number of other variables and comments from "grid" to
"surface", "gm" to "sm", etc.
Refactoring: Consolidated two large functions.

Consolidated the code from "test_many_bimolecular" and
"test_many_bimolecular_all_neighbors". These two large functions were
nearly identical and a flag is now used to indicate whether the
"all_neighbors" functionality should be used.

Additional refactoring should probably be done in react_cond.c to
minimize the incidences of copy-paste code.
Renamed GRID and MOL macros.

Renamed a number of macros that contained the words GRID and MOL to SURF
and VOL respectively. Also renamed a number of variables in a similar
fashion.