Allow providing a tck to compare to. Default to heuristic if empty

Closes #14 (closed)

ToDo:

• Check that comparison TCK is valid.
  • Cannot be self
  • Can be one of existing tcks
  • Can be one of the to-be-generated-within-MR tcks
  • Can be empty.
• Perform comparison with this passed TCK, else heuristic.

Successful job: https://gitlab.cern.ch/lhcb/tck-specs/-/jobs/41544505

STARTING SUMMARY OF JOB
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SUCCESS:: Created 'tmp/0x10101010_0x1000106c.diff' between the new tck, '0x10101010', and the reference tag, '0x1000106c
SUCCESS:: Created 'tmp/0x10101011_0x10101010.diff' between the new tck, '0x10101011', and the reference tag, '0x10101010
------------------------------------------------------------------
SUCCESS :: Generated TCKs
FINISHED SUMMARY OF JOB