Enhance plotting functions and documentation

[AbhirupaGhosh]

Updated documentation for plotPRC, plotROC, plotCM, plotDensity, and plotTopFeatsVI functions. Added new functions for plotting ROC curves, confusion matrices, and density of predicted class probabilities.

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• Feature (adds or updates new capabilities)
• Bug fix (fixes an issue).
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[epbrenner]

These work, but I'd strongly suggest that the import functions for the plots to natively include the tsv reading rather than requiring it.