Added script to (re)generate the virtual snapshot file for a multi-file snapshot.

What the name says. I tested this on a two file EAGLE 6 snapshot and h5diff returned no differences between the original virtual file and the one generated by this script.

Should be useful for snapshots that were created before this feature was part of SWIFT (I was told this is the case for SIBELIUS).

requested review from @matthieu

assigned to @bvandenbroucke

@rttw52 would that work on the Sibelius snapshots?

added python label

merged

mentioned in commit d2d3f6d6

Tested it and ran smoothly. Will be useful to have indeed. Thanks.

@jch might also be interested for some of the older runs.