add feature to compare "criticality" and sentiments between different teams

[milanwiedemann]

this is primarily for visualisations, but could also be relevant for text

[milanwiedemann]

also see #22 we need better word for criticality

[milanwiedemann]

this feature depends on a robust implementation of #20

[ChrisBeeley]

I'm moving this to 0.3.0- I can't see it being ready in three weeks