Introduce positional prop on VisualAttributes

While i2b2 uses uniqe characters in the three-character sequence for
ontology visual attributes, transmart, when adding new visual
attributes, repeats characters. Namely 'H', which means hidden in i2b2
and occurs at the second position, can actually occur at the thirs
position in tranSMART, there meaning 'high-dimensional data'.
Merge remote-tracking branch 'transmart/master'