find the nearest locations to a given point
floc = FastLoc.new(:unit => :miles) # :kilometers or :nautical_miles
floc.add(latdeg,lngdeg,3)
floc.add(latdeg,lngdeg,4)
floc.add(latdeg,lngdeg,5)
floc.add(latdeg,lngdeg,6)
floc.nearest(latdeg,lngdeg) -> [5,3,4,6]