Be more flexible on data frame as input

[jhollist]

Currently, data frame input must have first two columns as "x" and "y".

Could easily have it check for expected column names (x/y, lat/long, latitude/longitud, easting/northing) and if none of those (or multiple ones of those) error. Also provide an argument for specifying a vector of length two with the x and y column names. Similar to how st_as_sf does it.