Retreive a list of NTS references at a given scale by bounding box.
Bounding box is in the form returned by
sp::bbox(). NTS References
all have a valid series component (e.g. "021"), but map area (e.g. "H")
and map sheet (e.g. "01") are not checked to make sure they exist.
A bounding box of lat/lon values in the form returned by
list object containing zero or more NTS References.