Validate hashes in PKL and/or CPL

[thomasheritage]

Some possible aspects to include in IMP validation (some of these might already be covered):

• Ensure that CPL hashes (if present) match PKL hashes
• Calculate the hashes of files and ensure they match the stated hashes
• Make hash calculation optional (at least for MXF files -- perhaps always hash XML files given they are small and much more likely to have been (accidentally) edited)
• Include a statement in the validation report about whether hash validation was carried out (and the result) -- this is useful to know whether pass or fail (particularly given the amount of time it can take)