ripc 0.3.0

• Restructured API calls so that ipc_get() is more robust by defaulting
  to requesting a CSV return from the API rather than JSON, allowing for simpler
  cleaning in all ipc_get_...() functions.
• Allow return_format to be specified as GeoJSON for ipc_get_areas(), directly
  loading in an sf object.
• Improved the cleaning of ipc_get_population() nested JSONs to be more robust
  to changes in the API.
• Set R-CMD-check GitHub Action to run weekly to check for changes to the API,
  since all function examples are re-run during the check.