Fixed problems with API testing

The reference diffusion constant was never removed from the API test
when it was removed from the rest of code.

Also, some missing includes were added.
Organized the API testing code.
Added API calls to create surface regions.
Added API calls to create releases on regions.

The redundancies between mcell_create_geometrical_release_site and
mcell_create_region_release could be consolidated.
Made existing_region a static function.