Add additional neurotransmitter info for neural connections

[mwatts15]

After #401, it was found that of the neural connection records in the Shterionov connectome dataset, 590 are not in our standard C. elegans connectome dataset from wormwiring.org. It was also found that 3032 connection records in wormwiring.org are not covered by the Shterionov data.

This task is to add neurotransmitter info for the remaining connection records.