Allow use of filenames instead of the special IOVs file

This is to allow something like:

path/to/cond/1234
path/to/cond/2000
path/to/cond/2350
etc.

This is more human-friendly, but we need at least an extra (empty) file to tell that the condition path is "path/to/cond" and not "path/to/cond/1234". Partitioning of the timeline could be achieved via something like

path/to/cond/1000/1234
path/to/cond/2000/2000
path/to/cond/2000/2350

Like this we loose the deduplication we can have with the current policy, but we could use symlinks:

path/to/cond/.pool/value1
path/to/cond/.pool/value2
path/to/cond/1000/1234 -> ../.pool/value1
path/to/cond/2000/2000 -> ../.pool/value2
path/to/cond/2000/2350 -> ../.pool/value1

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