Fix xpath expression for conditional codelist.

A leading slash needs to be added to the expression used in codelist.xml (like conditional helper):

```
  <codelist name="gmd:MD_ScopeCode"
            displayIf="ancestor::node()[
              name()='gmd:MD_Metadata' and
              contains(gmd:metadataStandardName/gco:CharacterString, 'MedSea')]">
    <entry>
```