Extensive use of attribute multiplicity > 1

[pahjbo]

The VO-DML definition of CAOM makes fairly extensive use of attributes with multiplicity >1 - in the 1.0 schematron this is flagged as STRONGLY DISCOURAGED, but with little explanation in the error message itself - In general the reason why it is a problem is that attributes are directly mapped to database columns and therefore cannot simply have multiplicity > 1 without some further mapping convention. In some of the cases the attribute type is a primitiveType and therefore there could be a convention that the multiple values are serialised to strings and concatenated with a separator.

In other cases the attribute type is a dataType (which in the multiplicity = 1 case just has its members mapped to columns), but is more problematic with mulitiplicity>1. The pragmatic solution in general is potentially to change the dataTypes used in this way to objectTypes and then use them in compositions rather than as attributes.