Add support for cartesian product of sets

[fsestini]

An example of relation that would be enabled by this feature is po-scl-ob, which is part of lob:

 let scl = loc
  | (M | DC.CVAU | IC) * (IC.IALLU | IC.IALLUIS)
  | (IC.IALLU | IC.IALLUIS) * (M | DC.CVAU | IC)

let po-scl-ob =
 [Exp & M]; (po & scl); [DC.CVAU]
 | [DC.CVAU]; (po & scl); [Exp & M]
 | [DC.CVAU]; (po & scl); [DC.CVAU]