Add functionality to read in shear/convergence cluster simulated maps

[combet]

Write example notebook to read in the shear and convergence maps from the ANL high-res cluster simulations. Starting point in the old clmassmod repo, where MXXL sims were read in.

[marina-ricci]

This issue is old, but we still want to be able to deal with sims. I rephrased the title accordingly.

[marina-ricci]

@chrisdavies234 I think you can add your modules to solve this issue in a new PR.