[DDI-SRG] Missing value use case

[Pascal Heus]

All:
I've been exchanging email with a R user/developer in Italy who is 
integrating DDI into R (as part of an SPSS reader plugin). He needs to 
express a missing value range on a continuous variable (like all values 
between 3 and 5). I think we explicitly excluded this option but we may 
want to revisit since we may have use cases for this (gathering more 
info). I think we could address this by adding a isMissing attribute to 
the NumericRepresentation/NumberRange element.
*P