Adding calculations for more indices like DCP, NST, and TTS.

[alluringstorms]

What should we add?

I'm not sure how to do it myself but I thought some other people using MetPy might like it so I decided to suggest it here. Basically the adding of newer and some lesser known severe weather parameters for metpy.calc. I've left the references to the SPC's definition and equations in the GitHub References area below.

Reference

Some example indices that could be added to MetPy/indices.py are below:

Derecho Composite Parameter (DCP): https://www.spc.noaa.gov/exper/mesoanalysis/help/help_dcp.html
Non-Supercell Tornado Parameter (NST): https://www.spc.noaa.gov/exper/mesoanalysis/help/help_nstp.html
Tornadic Tilting and Stretching Parameter (TTS): https://www.spc.noaa.gov/exper/mesoanalysis/help/help_tts.html