python: relabel() and relabel_bse() should return the relabeling maps
One reason for relabeling a formula is if it must be passed to some tool that has a stricter syntax for atomic propositions. In that case we relabel all atomic proposition, call the tool, and do the reverse relabeling on the result.
Currently the python interface to
relabel_bse() returns a relabeled formula, but lose the information about what was renamed to what. We should definitely return this information, either as a Python
dict, or as a black-box object that can be passed to another function to relabel in the oposite direction.