initial work on popcons -- close to functional

[fsciortino]

Adding functionality to create Plasma OPeration CONtour (POPCON) plots, based on the work of the 22.63 MIT/Princeton class of fall 2020. Adapted Open_POPCON scripts to use atomic rates from ADAS via Aurora and Hale-Bosch reactivity already coded in omfit_classes.

The script in Aurora/examples/scan_popcons.py illustrates how POPCONs can be created and can be used for testing.

Work in progress.

@anelson0528 @sfrnk

[fsciortino]

@anelson0528 @sfrnk There is something funny about how profiles are being treated at the moment... and I haven't had time to deal with it yet. I've added some code that uses Aurora to find profiles of impurity charge state fractional abundances, so that the average Z for a given impurity species at each value of Te can be determined. If one sets a fixed impurity concentration value and uses these fractional abundances, then Zeff will not be flat over the radius. The code needs to be updated to deal with this.

I don't understand why functions like _P_rad are multiplied by rho at the very end. It sounds like an approximation for the specific parametrization of kinetic profiles, but I'm not sure that I follow why that's a good idea...

I'd appreciate if you could have a look and make any adjustments that you see fit. Thanks!