Add molecular phenotypes to Ivy neurons

[memartone]

According to definition, there are several distinguishing molecular species but we currently only list GABA:

definition "Ivy cells are highly abundant GABAergic interneurons when compared to basket, bistratified, or axo-axonic cells. "Ivy" cells are named after their dense and fine axons innervating mostly basal and oblique pyramidal cell dendrites. They express nitric oxide synthase, neuropeptide Y, and high levels of GABAa receptor aplha1 subunit and they are also identified as slow-spiking interneurons that regulate the excitability of pyramidal cell dendrites through slowly rising and decaying GABAergic inputs."