Subsets ws_monitor to include only those monitors (or grid cells)
within a certain radius of a target location. If no monitors (or grid cells) fall
within the specified radius, ws_monitor$data and ws_monitor$meta
are set to NULL.
When count is used, a ws_monitor object is created containing up to
count monitors, ordered by increasing distance from the target location. Thus, note that the number
of monitors (or grid cells) returned may be less than the specified count value if fewer than
count monitors (or grid cells) are found within the specified radius of the target location.