Define and assign surface classes via API.

The only surface class properties available right now are the ones that
let you release surface molecules by number or density. Reflective,
transparent, absorptive, and clamp concentration properties still need
to be added.
Define properties for surface classes via API.

Reflective, absorptive, and transparent surface class properties can be
defined via the API now. The concentration clamp property still needs to
be added. The wording of some functions (e.g.
mcell_add_surf_class_properties) may need to be adjusted so they are
consistent with the QRG, which considers any of the commands (including
MOLECULE_DENSITY/NUMBER) defined in a surface class to be a "surface
property".