Adding covariance calculation for uncorrelated large scale structures

[calumhrmurray]

Add code to calculate the covariance contribution from uncorrelated large scale structures in the field of galaxy clusters. The expression is given in equation 17 of Mclintock: https://arxiv.org/pdf/1805.00039.pdf although it seems to have a typo as the spherical bessel functions should be averaged in annuli (perhaps not a large difference). The correct expression is in equation 15 of Umetsu et al.: https://arxiv.org/pdf/1105.0444.pdf although one should change the 0->2 in the case of shear rather than convergence.