Reported by Nicola Fontana, Jun 15, 2010

In a lot of situations, AdgEntity should fail gracefully instead of 
complain for insufficient data.

For example, a linear dimension without the needed AdgNamedPair data 
should simply not be rendered (and no warnings should be raised). It 
is quite common to have a model with optional machinings: when the 
machinings are not performed, the named pairs are not defined and 
the dimension must simply disappear from the drawing.

After this has been implemented, the model on adg-demo must be fully 
implemented by adding the missing logic.