Added Hopkins 2013 formalism definitions in text, as well as fixed Eqn. 63…

Added Hopkins 2013 formalism definitions in text, as well as fixed Eqn. 63 (f_{ij} for Pressure-Entropy) as it was unclear.

There is some conversation to be had here about the formalism that is used in this documentation. I think sticking to defining x, \tilde{x}, y, and \tilde{y} and then just following the equations through would be clearer. That is something for another time, though.

Having it uniform over the document is better, yes. It could also be good to introduce the variables we actually use in the code

\tilde{\rho}

for instance.

BTW, you should update your master. Not important for these changes, but the rest is rather old.

merged

mentioned in commit 14b59fb5