Issue #51: Unit Type relation Population

The relationship between the Variable and the Unit Type is a bit problematic. It is difficult to restrict the Variable to measure only one Unit Type. This is especially difficult in the case of compiled data sets which often include compilation Variables that can be seen to include more than one Unit Type. For example, the Variable “Mothers Country of Birth” which can be seen to include Unit Types “Person” and “Mother”. Another example would be "Household's Reference person’s Age". A possible solution could be to add a relationship from Unit Type to itself, thus making the use of these information objects more flexible.