Discrete groups

[ncarazon]

Closes #3423

This PR enables discrete groups. I've tested question creation, forecast creation and withdraw. Everything worked as expected.

[lsabor]

Unfortunately, it doesn't quite work.
The first added subquestion works properly, but when you add an additional subquestion, it copies directly the nominal "range_min" and "range_max" . This was annoying enough to implement, and was the main reason why I didn't just launch them at the time.

After creation, the first one has range_min / range_max of -0.5 and 10.5 and inbound outcome count of 11 which are all correct,
but the second one has range_min / range_max of 0.0 and 10.0 and inbound outcome count of 10 which are all incorrect.

We need to make sure additional subquestions that copy from an earlier question copy the real value (-0.5) not the nominal value (0.0).

If this explanation isn't clear, i can go into more detail on a call.