Group data component cache files under a single directory

[mdpiper]

CSDMS now has several data components. Most cache data in a local directory to avoid fetching data that has already been downloaded; e.g., the Topography data component uses the directory ~/.bmi-topography. Instead of having multiple hidden directories in a user's home directory (one for each data component), we should group the cache directories under one hidden directory, ~/.csdms. The cache directory for the Topography component would then be ~/.csdms/bmi-topography; for GeoTiff, ~/.csdms/bmi-geotiff, etc.

This is issue is intended as a placeholder for all data components, and is motivated by a discussion in csdms/bmi-wavewatch3#4.