Draft implementation of http files

the Mode ‘URL’ has been added so viz ADCIRC files posted at URLs, and
not necessarily on THREDDS servers.

Call Example:

;AdcircViz('Mode','Url','Url','http://opendap.renci.org:1935/thredds/dod
sC/Test/DukeEnergy/MSL/trk04536_1/')