ss: Add utility method to increment an attribute by a value

ss: Add utility method to increment an attribute by a value

This new utility method is useful for analysis who need to increment
values of an attribute by a value that is not 1.

This can replace ITmfStateSystemBuilder#incrementAttribute().

Change-Id: I19037dda4c417d44e8f0aacc017545f921d18ff6
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/66944
Reviewed-by: Hudson CI