Sorting of attributes into Shared/PerFrame FGs

[fedorov]

@hackermd one issue we identified with the current legacy converter is that it does not detect attributes that are repeated and identical in the PerFrame functional groups to factor them out into Shared functional groups (which I think is non-compliant with the standard, and is a bug).

We want to fix this, and @afshinmessiah was going to work on this. But before he starts working on it, we wanted to confirm this contribution would be welcomed. If it is, then we would also appreciate if you tell us if you have specific approach in mind how and where you would want it to be implemented, or if you would want us to come up with a proposal.