facet_by: Specify a field to facet by
In hafen/finderquery: Utilities for Building EIOS Finder Queries

Specify a field to facet by

facet_by(
  query,
  field,
  limit = -1,
  sort = c("count", "index"),
  mincount = 0,
  offset = 0
)

`query`	a `query_facet()` object
`field`	field name or vector of two field names (see `facetable_fields()` for all possibilities)
`limit`	Controls how many constraints should be returned for each facet.
`sort`	Controls how faceted results are sorted.
`mincount`	Specifies the minimum counts required for a facet field to be included in the response.
`offset`	Specifies an offset into the facet results at which to begin displaying facets.