Changed convolve() to be a cpdef method in states module.
Changed convolve() to be a cpdef method in states module.

This is so that other modules (Python and Cython) have access to it as well.

pierrelb/RMG-Py,KEHANG/RMG-Py,chatelak/RMG-Py,jwallen/ChemPy,nyee/RMG-Py,pierrelb/RMG-Py,nickvandewiele/RMG-Py,KEHANG/RMG-Py,enochd/RMG-Py,chatelak/RMG-Py,comocheng/RMG-Py,nickvandewiele/RMG-Py,comocheng/RMG-Py,faribas/RMG-Py,enochd/RMG-Py,faribas/RMG-Py,nyee/RMG-Py