Add option to generate missing RepData files

[tobbee]

RepData files are files with information about all segments of a representation.
If a representation has such a side-loaded file, livesim2 can start much faster, since it does need to scan all segments to learn about their durations. The RepData files contain JSON data and are typically compressed using gzip and read an uncompressed as livesim2 starts up.

Currently, there is an option --writerepdata that needs to be used to create such RepData files. However, that will scan all representations and create new RepData. A common case, is that just some new assets have been added, so only the representations lacking RepData needs RepData generation.

In order to handle that case, a new option --writemissingrepdata should be implemented. By running that, one a make sure that RepData is only generated for the assets and representations needing it.

Since more fields are added to RepData over time, it makes sense to introduce a version number in RepData with a default of 0.
One can then add the functionality that RepData files that need to be updated are updated to the latest version.