Fixed Topography.read when argument is a URL, deprecated get_topo function and added improved version fetch_topo_url that is now used by read.  Added a unit test.
Fixed new unit test for fetch_topo_url to delete metadata file
Do not set .z attribute of Topograophy class unless its an unstructured grid
Moved and renamed tests/test_topotools.py (and data file).  Need to see if Travis works ok.