Add field to store file hash

[jokasimr]

It would be good for reproducibility if the File class had an optional file hash field. The file hash field could for example be a string formatted like md5:<hash-value> or sha1:<hash-value>.

Advantages:

• This would allow people to verify that the files they are working with are actually the same as the ones specified in a particular Orso file.
• In the beautiful future when we have content addressed file storage for scientific data researchers will be able to directly download the data referenced in an Orso file, provided of course that someone has at some point uploaded the data to the content addressed file repository.

[bmaranville]

I think we discussed this at one of our meetings, but I'm not sure. I can't think of a good reason not to include it. Would have to support sha256 etc. as well...