IEV improvements: an entry can have a definition, or non-verbal representation, or both

> According to ISO 10241-1:2011, Table 1 and 6.5, a terminological entry can have either a definition or a non-verbal representation, or both.
> As per ISO 10241-1:2011, 6.5, non-verbal representations can be used to exemplify the concept … and usually comprise visual representations such as figures (e.g. photographs and pictographic representations, technical drawings, charts, graphs, diagrams,
etc.), tables and mathematical expressions. In some non-verbal representations, colour is semantically significant. In a multilingual standard, it is recommended to use language-independent non-verbal representations.

* Examples of non-verbal representations are provided in ISO 10241-1:2011
* A.2.19 Examples showing non-verbal representations within definitions, and grammatical information
* A.2.20 Example showing a non-verbal representation within a note.

ISO 10241-1:2011 provides contradictory information regarding whether there can be more than one non-verbal representation:
* Table 1 says "non-repeatable" (i.e. occurrence ?)
* 6.5 clearly allows more than one ("If more than one …") (i.e. occurrence *)

We should model it as:
* concept has definition (or not)
* concept has nonVerbalRep*