use @base when present

[VladimirAlexiev]

The input rdfxml uses xml:base:

<?xml version="1.0" encoding="UTF-8"?>
<rdf:RDF
    xml:base="http://fullgrid.eu/CGMES/3.0"
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:eu="http://iec.ch/TC57/CIM100-European#"
    xmlns:md="http://iec.ch/TC57/61970-552/ModelDescription/1#"
    xmlns:cim="http://iec.ch/TC57/CIM100#" > 

  <cim:Discrete rdf:ID="_13dacabf-aa4c-4a78-806e-c7c4c6949718">
    <cim:Measurement.unitSymbol rdf:resource="http://iec.ch/TC57/CIM100#UnitSymbol.none"/>
    <cim:IdentifiedObject.mRID>13dacabf-aa4c-4a78-806e-c7c4c6949718</cim:IdentifiedObject.mRID>
    <cim:Measurement.PowerSystemResource rdf:resource="#_0d76d781-5117-4e1c-af35-ca8849474b39"/>
    <cim:IdentifiedObject.description>DIS_1</cim:IdentifiedObject.description>
    <cim:Measurement.measurementType>SwitchPosition</cim:Measurement.measurementType>
    <cim:Discrete.ValueAliasSet rdf:resource="#_1a457323-2094-440f-8d30-dc93adf0cdb3"/>
    <cim:IdentifiedObject.name>DIS_1</cim:IdentifiedObject.name>
    <cim:Measurement.unitMultiplier rdf:resource="http://iec.ch/TC57/CIM100#UnitMultiplier.none"/>
    <cim:Measurement.phases rdf:resource="http://iec.ch/TC57/CIM100#PhaseCode.ABC"/>
  </cim:Discrete>

owl correctly interprets the base, but outputs Turtle that doesn't use base, making it significantly longer:

owl.bat write -i rdfxml FullGrid_OP-with-base.rdf
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix cim: <http://iec.ch/TC57/CIM100#> .
@prefix md: <http://iec.ch/TC57/61970-552/ModelDescription/1#> .

<http://fullgrid.eu/CGMES/3.0#_13dacabf-aa4c-4a78-806e-c7c4c6949718> a cim:Discrete ;
  cim:Discrete.ValueAliasSet <http://fullgrid.eu/CGMES/3.0#_1a457323-2094-440f-8d30-dc93adf0cdb3> ;
  cim:IdentifiedObject.description "DIS_1" ;
  cim:IdentifiedObject.mRID "13dacabf-aa4c-4a78-806e-c7c4c6949718" ;
  cim:IdentifiedObject.name "DIS_1" ;
  cim:Measurement.PowerSystemResource <http://fullgrid.eu/CGMES/3.0#_0d76d781-5117-4e1c-af35-ca8849474b39> ;
  cim:Measurement.measurementType "SwitchPosition" ;
  cim:Measurement.phases cim:PhaseCode.ABC ;
  cim:Measurement.unitMultiplier cim:UnitMultiplier.none ;
  cim:Measurement.unitSymbol cim:UnitSymbol.none .

Let me know if you want a test file