Added API function to create new species.

The API function mcell_create_species allows one to create/define new
species without the use of the parser. The same functions are used by
both the parser and the API to minimize redundant code.

Currently, the API function call needs to come after mcell_init_state
and before mcell_init_simulation. Also, it needs to be called after the
time step has been set. Perhaps, the ordering dependency can be loosened
somewhat in the future.

Lastly, it might be nice to add a simplified version of
mcell_create_species, which doesn't require passing the "optional"
molecule commands, like custom time step, target only, or maximum step
length.