enhancements to matrix_report.py to be better for SSNs

[seanrjohnson]

matrix_report.py currently reports all datapoints in the matrix, including the diagonal. For symmetric matrixes intended for defining edges of SSNs, it would be nice to filter the data so that the diagonals are ignored, and only the lower or upper triangular is considered.

It would also be nice to have more SSN-specific plots, like number of clusters that would result from various score cutoffs, average/median connectivity from various score cutoffs. Perhaps there are other metrics that would be informative.