Feature/electron models

[vedantdhruv96]

Touched up the PR by @blancocd.

What's new:

• Implements multiple electron-heating models - Kawazura (2019), Werner (2016), Rowan (2017) and Sharma (2007). TheELECTRONS flag in parameters.h runs the Kawazura model by default. Setting ALLMODELS to 1 heats electrons independently according to the four models mentioned above. NOTE: If all electron models are simulated, there will a total of 13 primitives: RHO, UU, U1, U2, U3, B1, B2, B3, KTOT, KEL0, KEL1, KEL2, KEL3 where KEL# stands for electron entropy and 0, 1, 2, 3 => KAWAZURA, WERNER, ROWAN, SHARMA respectively.
• Equations are cited for the formulae used to partition total dissipation into heating electrons (and ions).
• HOWES model omitted

[bprather]

I like this way of doing things, but note that if we ever change the order of KEL values, it will break any subsequent imaging work in ipole -- thus we should add them to the output spec here.

To make my life easier when trying to image all these options in ipole, can we bump the version in decs.h to "iharm-alpha-3.7"? That way we'll be able to distinguish easily what ordering to use when trying to read electron temperatures.

(Honestly we should have the master branch using "release" or at least "beta" to distinguish, but that's for another time.)