Check that input theories correspond to a prescription

Created by: RosalynLP

@Zaharid @lucarottoli @voisey I implemented the check on the xi_R and xi_F values of the input theories which Luca suggested. Interestingly enough when I first ran it it threw an error for a test runcard I'd been using and when I looked into it it turned out that I'd actually messed up the theory inputs for that runcard! So it was clearly a useful thing to do :)