extract_trips: Extract trips from scenario
In tlumip/swimr: Functions for Looking at Swim

This function looks at the trip matrix for a scenario and dplyr::summarizes trips on the origin end by mode and region of origin.

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extract_trips(db, facet_var = NULL, facet_levels = NULL,
  color_levels = c("auto", "transit", "non-motorized", "truck"),
  index = FALSE)

`db`	the scenario database.
`facet_var`	The region to dplyr::summarize by.
`facet_levels`	Regions to include in summary.
`color_levels`	Modes to include in summary. Defaults to all modes other than `school`. See consolidated modes in mode_types.
`index`	whether to index mode split off the base year.