Disambiguate between two LFN downloading methods

[alexpearce]

The "downloading data" lessons gives two ways to download LFNs that are in a Python file downloaded from the bookkeeping: using a custom Python script that needs the LFNs pasted into it, or using dirac-dms-get-file.

This isn't in the spirit of things, we should generally show The One True Way of doing things, or at the very least strongly recommend one technique over another. IMO we should just have the dirac-dms command.