Update grayscaleTodBZ docstring.
Add a dictionary for layer bounding boxes in fmidata.py.
Update getRadarImage in fmidata.py.

The default image format is changed to png, since
geotiff doesn't seem to work. The function now
also uses the boundingBoxes dictionary. Right now
it gives an error, if no bounding box is found in
the dictionary nor supplied in the parameters, but
this will be fixed later.