Support numeric vs char-based attributes

[gmichalo]

We need to allow users to define the attribute types of the columns of the Initial dataset. In addition, we need to take into consideration the type of the attributes when we use DCs.

Benefit: Holoclean will support numeric values attributes

Cost: 1-2 days. We need to change the way we create the Initial dataset. In addition, we need to change the possible values table in order to support numeric values