Overriding pooch cache location

[willirath]

On many HPC systems, $HOME has a pretty small quota. So we likely want to make it easy for the user to override this.

There does not seem to be a way to configure a base dir via ENV vars. So going for something along the lines of

mypooch = pooch.create(
    path=<from cli arg?>,
    ...
)
mypooch.retrieve(...)

should be the way to go.
See https://github.com/fatiando/pooch/blob/3339116472e9ce3ee5d47ef562e29cb41e24797f/doc/user-defined-cache.rst