add optional content hash to File object

[bmaranville]

In addition to timestamp, it would be nice for reproducibility if we could specify a hash of the file contents for File objects, which are used in the Measurement class.

[arm61]

Sounds like a good feature to me.

[aglavic]

I think it's a good idea.

Some suggestions/comments:

1. Add a test for the new class.
2. Hashes are, by default, binary strings. How would we deal with that? Is it compatible with YAML? (Or did you plan to use the hexdigest representation, in which case the attribute should maybe be renamed.)
3. Add a convenience method in the ContentHash class to check a string against the given hash digest.
4. Add a convenience class mathod to ContentHash create the object from the test string automatically.