rdf2sparql: interpolate variable in prefixed URL

[VladimirAlexiev]

It is possible to interpolate variables (and invoke macros) in templated URLs:

<schema/refinery/(n)> a csvw:Column, sosa:ObservableProperty;  ## have it
  csvw:datatype <schema/refinery/(n)/(dtBase)>, "(datatype)";  ## have it
  sosa:hasFeatureOfInterest <feature/URLIFY(SPLIT(features))>;  ## have it
  un:qualifier <qualifier/SPLIT(statisticalQualifiers)>;   ## have it
  qudt:hasQuantityKind qk:\(quantityKind\);  ## need it 
  qudt:hasUnit unit:\(unit\);  ## need it

However, there's also a need to interpolate vars in prefixed URLs (last 2 lines above).

Notes:

• I could write this as as follows, but I prefer not to use full URLs, to avoid mistakes

<http://qudt.org/vocab/unit/(unit)>
<http://qudt.org/schema/quantityKind/(quantityKind)>

• Parentheses need to be escaped in a prefixed URLs but they display fine in rdfpuml
  
• for now interpolating single vars is enough