Add an option to run the analysis on local data

[eguiraud]

For the purpose of testing different setups and configurations it is often useful to be able to run the analysis workload with data on local storage (e.g. to cut network I/O, xrootd caches etc. out of the picture).

It would be nice if the AGC implementations had an option to pre-download a small set of files to the specified local path and do a run on that data instead.