find all blockgroups at least partly in bounds of city/cities specified

Usage

fips_bgs_in_city(fips = testinput_fips_cities[1:2], approx = TRUE)

fips: vector of city/CDP/town fips as from among censusplaces$fips
approx: optional, set to FALSE if you need exactly which blockgroups overlap at all with city/cities, but that method is much slower as it downloads all blockgroup boundaries for all relevant counties containing specified cities. The approx method finds all blockgroups for which at least one block centroid is inside the city polygon. It is MUCH faster, but can sometimes leave out a blockgroup that only slightly overlaps the city.

vector of blockgroup fips codes

used by fips_bgs_in_fips(), and uses fips_bgs_intersect_city_approx() and fips_bgs_intersect_city_exact() helpers