Add "latLonBounds" keyword to loadFile

Currently expected format is (leftLat, leftLon, rightLat, rightLon)
(syntax suggestions welcome)

e.g. latLonBounds = (30.0, -80.0, 35.0, -73.0)

[1589]
Add "transparency" keyword to "setEnhancementTable" method, e.g.:

layer.setEnhancementTable('VisAD', transparency=0.5)

where transparency is between 0 and 1.

[1597]