Current CSV export is based on XSL transformations and can be hard to use when user is interested in element with multiple values (eg. online source, contacts).

Default CSV export remains the same but add the possibility to customize the export with 2 additional parameters to create custom export:

* `loopElementXpath`: element to loop on eg. use
`.` for the metadata
`.//gmd:CI_ResponsibleParty` for all contacts in ISO19139,
`.//gmd:transferOptions/*/gmd:onLine/*` for all online resources in ISO19139.

* `propertiesXpath`: columns to extract eg.
`gmd:identificationInfo/*/gmd:citation/*/gmd:title//text()` for the title.

Also add parameters for defining separators:
* `sep` for column separator
* `internalSep` when multiple values are stored in a field

User can then build custom reports from the API:

eg. Export Contact with role,org,email
http://localhost:8080/srv/api/records/csv?bucket=s101&loopElementXpath=.//gmd:CI_ResponsibleParty&propertiesXpath=gmd:role/*/@codeListValue&propertiesXpath=gmd:organisationName/*/text()&propertiesXpath=.//gmd:electronicMailAddress/*/text()

eg. Export Online source with protocol,url,name,desc
http://localhost:8080/srv/api/records/csv?bucket=s101&loopElementXpath=.//gmd:transferOptions/*/gmd:onLine/*&propertiesXpath=gmd:protocol/*/text()&propertiesXpath=gmd:linkage/*/text()&propertiesXpath=gmd:name/*/text()&propertiesXpath=gmd:description/*/text()

eg. Export Metadata with title,alternateTitle,status,maintenanceFreq,...
http://localhost:8080/srv/api/records/csv?bucket=s101&loopElementXpath=.&propertiesXpath=gmd:identificationInfo/*/gmd:citation/*/gmd:title//text()&propertiesXpath=gmd:identificationInfo/*/gmd:citation/*/gmd:alternateTitle//text()&propertiesXpath=gmd:identificationInfo/*/gmd:status/*/@codeListValue&propertiesXpath=gmd:identificationInfo/*//gmd:maintenanceAndUpdateFrequency/*/@codeListValue&propertiesXpath=gmd:identificationInfo/*//gmd:otherConstraints//text()&propertiesXpath=gmd:identificationInfo/*/gmd:topicCategory//text()&propertiesXpath=gmd:identificationInfo/*/gmd:language/*/@codeListValue&propertiesXpath=gmd:identificationInfo/*/gmd:graphicOverview/*/gmd:fileName/*/text()

Functions from XPath v1 can also be used eg. `count`
http://localhost:8080/srv/api/records/csv?bucket=s101&loopElementXpath=.&propertiesXpath=count(gmd:identificationInfo/*/gmd:descriptiveKeywords)

For all exports 2 columns are added first:
* UUID
* permalink

When using XPath, it is recommended to export records in same schema (or at least same base schema). If not, then XPath error messages are returned in cells unless XPath provided do not require namespaces. Users have to configure a proper selection to avoid mixing schema.

…enabled.