Enable arbitrary dependence constraints for ALTER GENERATOR with cgpm_backend

[fsaad]

The phrase

alter generator g0
                ensure variables <vars> dependent;

will currently fail unless <vars> is *, since earlier implementations of the CrossCat inference algorithm with (in)dependence constraints failed to correctly account for ergodicity issues, see probcomp/crosscat#76.

However, recent patches to cgpm/lovecat fixed this issue and support arbitrary dependence constraints with correct inference algorithm ensuring ergodicity of the MCMC chain (independence constraints are still tricky to handle formally and therefore will remain disabled for now).