Change the Sigma(R) call to not need NFW parameters

[tmcclintock]

Currently the function Sigma_at_R() takes in NFW parameters, despite taking a whole correlation function as input. The NFW parameters are not necessary unless R_perp < r, in which case a model must be assumed (such as NFW) below r[0].

The default behavior should get rid of needing NFW parameters and a method should be implemented to chose the model below the small scale.