Feature: Allow openairmaps to auto-detect certain limits (e.g., polarMap(limits = ...))

[jack-davison]

This PR implements #34.

In short, it changes up how limits and similar args are handled to allow polarMap() and friends to automatically detect limits.

devtools::load_all()
#> ℹ Loading openairmaps

# default behaviour
polarMapStatic(polar_data, "no2", limits = "free")
#> ℹ Assuming latitude is 'lat'
#> ℹ Assuming longitude is 'lon'
#> Creating Polar Markers ■■■■■■■■■■■■■■■■                  50% |  ETA:  2sCreating Polar Markers ■■■■■■■■■■■■■■■■■■■■■■■           75% |  ETA:  1s                                                                         ℹ Map tiles by Stamen Design, under CC BY 3.0. Data by OpenStreetMap, under ODbL.

# fix limits
polarMapStatic(polar_data, "no2", limits = "fixed")
#> ℹ Assuming latitude is 'lat'
#> ℹ Assuming longitude is 'lon'
#> Creating Polar Markers ■■■■■■■■■■■■■■■■                  50% |  ETA:  1sCreating Polar Markers ■■■■■■■■■■■■■■■■■■■■■■■           75% |  ETA:  1s                                                                         ℹ Map tiles by Stamen Design, under CC BY 3.0. Data by OpenStreetMap, under ODbL.

# specify limits
polarMapStatic(polar_data, "no2", limits = c(0, 100))
#> ℹ Assuming latitude is 'lat'
#> ℹ Assuming longitude is 'lon'
#> Creating Polar Markers ■■■■■■■■■■■■■■■■                  50% |  ETA:  2sCreating Polar Markers ■■■■■■■■■■■■■■■■■■■■■■■           75% |  ETA:  1s                                                                         ℹ Map tiles by Stamen Design, under CC BY 3.0. Data by OpenStreetMap, under ODbL.

^{Created on 2023-07-31 with reprex v2.0.2}