Equations module allowing for fairly general quintessence models by Antony Lewis (http://cosmologist.info/) FIX March 2005: corrected update to next treatment of tight coupling Fix Oct 2011: a^2 factor in ayprime(EV%w_ix+1) [thanks Raphael Flauger] Oct 2013: update for latest CAMB, thanks Nelson Lima, Martina Schwind May 2020: updated for CAMB 1.x+ Notes at http://antonylewis.com/notes/CAMB.pdf This module is not well tested, use at your own risk! Need to specify Vofphi function, and also initial_phi You may also need to change other things to get it to work with different types of quintessence model It works backwards, in that it assumes Omega_de is Omega_Q today, then does a binary search on the initial conditions to find what is required to give that Omega_Q today after evolution.