Metadata schema: properly constrain all kinds of values

[cspurk]

In the next version of the metadata schema, all kinds of values should be properly constrained.

For example, it makes no sense for height or weight under participantInfoType to have values of type xs:integer with a value space of at least 18 digits. Values of type xs:unsignedShort would be totally sufficient here. The values of ageRangeStart and ageRangeEnd would sufficiently be described with xs:unsignedByte values. And so on.

Another example where proper constraints would be helpful are enumerations of strings. Instead of xs:string, the type xs:token would probably better suited. This would also remedy problems like the trailing whitespace in the enumeration value of the current metadata schema.

[cspurk]

#527 can be seen as a special case of properly constraining values, too.