Add support for Topography/Bathymetry NetCDF Files from NGDC

[mandli]

The support for NetCDF files in GeoClaw assumes something about the names of variables in the files to identify the correct variables to read. Right now this fails if those assumed are not found. One example of a file that currently does not work can be downloaded from

https://www.ngdc.noaa.gov/metaview/page?xml=NOAA/NESDIS/NGDC/MGG/DEM/iso/xml/5230.xml&view=getDataView&header=none

[mandli]

Note that the Python version that read topography allows for setting the fields directly. We may either want to have a list of traditional values or allow an input that specifies these values.