Overriding cdef method with def method warning for make_object

[amarkpayne]

Bug Description

The following warnings pop up whenever we cythonize RMG during the make process.

[38/46] Cythonizing rmgpy/statmech/rotation.pyx
warning: rmgpy/statmech/rotation.pyx:87:4: Overriding cdef method with def method.
[39/46] Cythonizing rmgpy/statmech/schrodinger.pyx
[40/46] Cythonizing rmgpy/statmech/torsion.pyx
warning: rmgpy/statmech/torsion.pyx:93:4: Overriding cdef method with def method.
[41/46] Cythonizing rmgpy/statmech/translation.pyx
[42/46] Cythonizing rmgpy/statmech/vibration.pyx
[43/46] Cythonizing rmgpy/thermo/model.pyx
[44/46] Cythonizing rmgpy/thermo/nasa.pyx
warning: rmgpy/thermo/nasa.pyx:86:4: Overriding cdef method with def method.

These warnings all are due to the make_object and as_dict method. I think I just forgot to use cdef here. It should be a short fix.

How To Reproduce

Run make clean; make on current RMG-Py master (41f09da)

Expected Behavior

No warnings should be displayed