Import Species from species (to avoid Julia)

I think we just need the Species class which
we can import directly from rmgpy.species
instead of from rmgpy.rmg.model which
in turn imports it from rmgpy.species
(but which also imports a bunch of other stuff while loading).
This is part of a general campaign to reduce un-needed import dependencies
and perhaps make it so some partial imports
can avoid loading all the RMS julia stuff.

Author: rwest

rmgpy/rmg/main.py

@@ -52,7 +52,7 @@
 from scipy.optimize import brute
 
 import rmgpy.util as util
-from rmgpy.rmg.model import Species
+from rmgpy.species import Species
 from rmgpy.rmg.pdep import PDepNetwork
 from rmgpy import settings
 from rmgpy.chemkin import ChemkinWriter