Adding cluster count covariances into TJPCov

[mattkwiecien]

This PR adds a new CovarianceClusters implementation of the CovarianceBuilder class, which is then further subclassed by the CovarianceClusterCounts subclass.

• Added code from N. Ferreira's python implementation of the cluster count covariances into covariance_clusters.py
• Modified the CovarianceClusters class to (1) load in SACC file (2) read the min/max redshift/richness values and number of bins, and (3) use those values in the cluster covariance calculation.
• Added a mock sacc file that contains cluster tracers (generated using Celine's code), each tracer's metadata contains information about the richness and redshift bin edges.
• Added a jupyter notebook where we load in a sacc file, calculate the theoretical covariance, plot the result, and save the covariance back to sacc.

We still need to do some validation but the basic workflow is working.

[mattkwiecien]

Closing since this is now in a different PR #50