Handle Basevoltage for ACLineSegment

Many models in ModPowerSystem take nominal voltage as start value of their terminal voltage (if not specified), thus highly deviated nominal voltages can cause convergence problem during the initialization. Therefore, it would be better to also have a conversion from the BaseVoltage attribute of ACLineSegment into PiLine's (ModPowerSystem) Vnom attribute.

  <cim:BaseVoltage rdf:ID="_9c5bc444-47e2-8abe-fad4-22efc221de38">
    <cim:BaseVoltage.nominalVoltage>400.</cim:BaseVoltage.nominalVoltage>
    <cim:IdentifiedObject.description>BaseVoltage 400.00 kV</cim:IdentifiedObject.description>
    <cim:IdentifiedObject.name>400.00 kV</cim:IdentifiedObject.name>
    <entsoe:IdentifiedObject.shortName>400.00 kV</entsoe:IdentifiedObject.shortName>
  </cim:BaseVoltage>

   <cim:ACLineSegment rdf:ID="_877e4bc7-caf6-44d6-8ab6-91620bb892f1">
    ...
    <cim:ConductingEquipment.BaseVoltage rdf:resource="#_9c5bc444-47e2-8abe-fad4-22efc221de38" />
    ...
  </cim:ACLineSegment>