Add support for frequency data in egg output streams

[laroque]

There are various cases, but in general it would be useful to be able to include frequency data in addition to/instead of time data. It is worth spending some time thinking about how we would actually want this to look.

The current use case is for validation of psyllid nodes, though this may be the final desired data product in future scenarios.

[nsoblath]

See the standards proposal in #31.