Added omicron, based on 3 early studies.

Preliminary inclusion of Omicron variant, based on 3 sources (preprints for the moment), as discussed in dev team meeting 5/1/22.

The user guide points to the issue (#226 (closed)), which I propose to keep open until we have an authoritative source on the relative R value (ideally a metastudy, as we have for Delta).

Omicron is now the new default, reflecting prevalence across most of Western Europe.