Add shim for gfal2 interface using subprocess

[riga]

In some environments, gfal2 and its python bindings might require manual hacks to run. It might be useful to have a shim that transfers the gfal2 API calls to a subprocess running the python bindings in a customizable environment.

@yrath

[riga]

The remote file interface has been abstracted in 0bc7e34, and specialized again in the GFALFileInterface.
The next step would be to create an other interface that uses a running subprocess.