create a helper function to compute checksums for all 'Data File' objects

[proccaserra]

• [x] use hashlib
• [x] use file uri as input
• [x] request checksum type from user (default to sha2 or md5)
• [x] add 2 Comments[] to each ISA DataFile object
  • [ ] Comment[checksum type]
  • [ ] Comment[Checksum]
• [ ] include a flag in the serialization function to request if checksums should be generated or recalculated (avoid to recompute checksum everytime isa objects are serialized