Add general Chi2 script and README

[02-infinity]

To Evaluate Fit Performance of the model: We have taken the data and best fit MC with six parameters (using nuismin) involving the sum of Diagonal Covariance Matrix and off diagonal elements of PCA1 matrix but instead of using the covariance from the same file, we have used the full covariance matrix from the nominal nuiscomp file. So basically, “External” matrix is used to compute chi-square for the data-MC difference vector, instead of the “internal” covariance from the same file as data-MC. This code and README file is all about that.
Data/MC: MicroBooNE CC1u1p (Muon Momentum, MuonCosTheta, Proton momentum, ProtonCosTheta)
GENIE + AR23 tune

[luketpickering]

Hi Shivanshi, I'm not entirely convinced this needs to be committed to NUISANCE itself. What is the use case for it? It computes a chi2 between a data and a prediction MC with an arbitrary matrix? This seems like a user feature that can be expressed in a few lines of ROOT linear algebra in user analysis. What is the general use case?

If there is one we are happy to accept the PR, but it's format needs to change, the new fils ot merge are currently not in a standard location or included in the buildsystem and the readme is not super clear.