add PCA plots for interindividual variability

[efigb]

Add PCA plots aggregating data based on mutation density and colored by defined groups, and store it in plots/interindividual_variability output directory in a separated pdf file.
Code can be added in ~/deepCSA/bin/plot_explore_variability.py.

[FerriolCalvet]

postponed for now, since we need to see exactly how is the best way to handle this
I just removed the branch that was created to tackle this