Positive potential derivative

[Jobilthomas]

Hey, I was working with a hybrid inflation model with a positive derivative of potential (dV/dphi). Since CLASS is written exclusively for negative potential I had to find and modify for my positive derivative model.
Here in this pull request I have generalized the code so that the code will work for both positive and negative potential derivative. Also it is done on top of the "primordial-bug-fix" pull request that I did recently.
Changes

• Added a new parameter, "potential_derivative", so that users can specify whether derivative is positive or negative (default = negative)

• Modified primordial_inflation_evolve_background

  1. added sign_der = +1 or -1 for case phi
  2. replaced while (sign_dtau*(quantity - stop) < 0.) with while (sign_dersign_dtau(quantity - stop) < 0.)

• Modified primordial_inflation_check_potential

• Modified primordial_inflation_find_phi_pivot