function getDataSource (source) {
  let geoJson
  if (source.source === 'overpass') {
    geoJson = require(getSourceDownloadName(source.id))
  } else if (source.source === 'manual-polygon') {
    geoJson = polygon(source.data).geometry
  } else if (source.source === 'manual-multipolygon') {
    geoJson = multiPolygon(source.data).geometry
  } else if (source.source === 'final') {
    geoJson = require(getFinalTzOutputFilename(source.id))
  } else if (source.source === 'final1970') {
    geoJson = require(getFinal1970TzOutputFilename(source.id))
  } else if (source.source === 'finalNow') {
    geoJson = require(getFinalNowTzOutputFilename(source.id))
  } else {
    const err = new Error('unknown source: ' + source.source)
    throw err
  }
  return geoJsonToGeom(geoJson)
}
Get the geometry of the requested source data

@return {Object} geom  The geometry of the source
@param {Object} source  An object representing the data source
  must have `source` key and then either:
    - `id` if from a file
    - `id` if from a file