Remove the Line from the ConstituentVxF model that Ignores the vxfref Property

The ConstituentVxF model class is currently ignoring the vxfref property from that class using the JsonIgnoreProperties property. The problem surges when one wants to retrieve/read that property from a ConstituentVxF object, which in this case is needed for the feature #31.

The fix can be done by removing the JsonIgnoreProperties line.