added most2geoclaw.get_comMIT_topo function to grab topo file from web, but doesn't always work!  This module needs refactoring.
Fixed default rigidity mu=4e10 not 4e11 since standard units are now Pa
Topography.read doesn't handle http addresses
set _X and _Y to None in Topography.read so if a new file is read it, X and Y will be regenerated when required.
Fixed maketopo.py file in chile2010 example for latest tools and to also plot if requested.