Fix checkpointing for relaxed clock models

[GuyBaele]

Checkpointing for relaxed clock models is not being done correctly.
While a checkpointed run will resume without problems, the exact trace of states will not be reconstructed as the entries of the branchRates.categories parameter will have changed (or will have been permuted) as a result of an internal mapping procedure.
As a result, even though the branchRates.categories parameter is first restored correctly, it is later overwritten in the adoptTraitData method and hence subsequent calls to e.g. uniformIntegerOperator will immediately diverge compared to a longer run without checkpointing.