Covariance matrix of shear profile for single cluster

[payerne]

Create a new functionality of the galaxycluster object, for the estimation of the shear profile covariance matrix. I have an example here that uses jackknife resampling of background galaxies for the estimation of the covariance matrix, with CLMM mocks. (it works similar to the stacked covariance matrix with Jackknife, but this time this is the background galaxy that are resampled instead of the galaxy clusters in the stack)

[payerne]

Issue attached to branch #505, where I put the Jupyter notebook in the example folder.