Add location's accuracy_radius

[RCura]

This field is present in MaxMind cities DB (both the free GeoLite2 City and the proprietary GeoIP City databases). Unfortunately, I could'nt add it to the test_that tests, considering that the current example DBs inside inst/ do not contain this field. I tested it both with a proprietary MM GeoIP2 City and the free GeoLite2 City db, and it worked in both case for the already tested adress.