start cache clear ops

[martindurant]

POC for #325
@gmaze , this is not quite what you need, but I though would be useful for the general user. What do you think?

[gmaze]

awesome !
This should provide a minimal way of clearing the cache

Is the os.remove(fn) ok and won't trigger an error like #322 ?

[martindurant]

Is the os.remove(fn) ok and won't trigger an error like #322 ?

If the cache dir is writable, we should be able to do this.

[gmaze]

Thanks a lot @martindurant for being so fast with this !
I'll be able to use this in https://github.com/euroargodev/argopy right away