Add neon bulb model

[diimdeep]

• https://en.wikipedia.org/wiki/Neon_lamp
• https://www.bristolwatch.com/ele/neon.htm
• http://ltwiki.org/files/LTspiceIV/lib/sub/neonbulb.sub

[ipatrol]

I think the especially useful aspect of this would be modeling the negative resistance curve, which cannot be done with just the existing spark gap model, but is a common feature of gas-filled tubes.

[pfalstad]

hmm, if so, then that neonbulb.sub model doesn't look sufficient.