feat(data/set/basic): add `set.set_ite` (#6557)
I'm going to use it as `source` and `target` in
`local_equiv.piecewise` and `local_homeomorph.piecewise`. There are
many non-defeq ways to define this set and I think that it's better to
have a name than to ensure that we always use the same formula.
Co-authored-by: sgouezel <sebastien.gouezel@univ-rennes1.fr>
