extract_employment: Extract employment statistics from scenario
In tlumip/swimr: Functions for Looking at Swim

This is an internal function to pull the employment data from a scenario and return it to either the single scenario or scenario comparison functions.

1 2	extract_employment(db, facet_var = NULL, facet_levels = NULL, type_levels = NULL, employment_categories = NULL)

`db`	The scenario database.
`facet_var`	The variable in the zone table to facet by. Defaults to MPO
`facet_levels`	The levels of the facet variable to keep. Defaults to all levels other than external stations.
`type_levels`	The types of employment to show in the plot.
`employment_categories`	[Optional] data_frame of employment aggregation categories; if `NULL`, uses `emp_types`. Should have columns `ACTIVITY` (full code of activity in AA databases) and `emp_type`.