Add subcommand to clean cache files

[mcflugen]

This pull request adds the clean subcommand to ww3 that remove cached data files.

The following removes all cached data files from the default location (~/.wavewatch3/data)

$ ww3 clean

A user can specify another location through the --cache-dir option.

[mdpiper]

I wonder if we should use a top-level directory (like ~/.csdms) to group the cache files for our data components? So we'd have ~/.csdms/wavewatch3, ~/.csdms/bmi-topography, etc.

[mcflugen]

@mdpiper I was wondering the same thing. I don't have a strong opinion at the moment but it's something worth thinking about.

[mdpiper]

@mcflugen I'll add a placeholder issue to https://github.com/csdms/bmi-topography (since I'm less likely to forget about it there).