CSW / Make the distinction between 'own' and 'http://www.isotc211.org/2005/gmd' for outputSchema.

When adding a new schema (eg. ISO19115-1), user may require backward compatibility with ISO19139. In that case CSW request using 'csw:IsoRecord' or 'http://www.isotc211.org/2005/gmd' should return the record in ISO19139:
```
<?xml version="1.0"?>
<csw:GetRecordById xmlns:csw="http://www.opengis.net/cat/csw/2.0.2"
  service="CSW"
  version="2.0.2"
  outputSchema="http://www.isotc211.org/2005/gmd">
    <csw:Id>cecd1ebf-719e-4b1f-b6a7-86c17ed02c62</csw:Id>
    <csw:ElementSetName>brief</csw:ElementSetName>
</csw:GetRecordById>
```

If user wants to retrieve the original document in its own schema, then 'own' outputSchema could be used.

For a schema plugin which should return the same document in both cases (like iso19139), copy the iso19139 schema csw/own-*.xsl.


At a later stage, it may be relevant to register outputSchema from profile configuration (see https://github.com/geonetwork/core-geonetwork/blob/develop/csw-server/src/main/java/org/fao/geonet/csw/common/OutputSchema.java#L21). For exemple, one namespace in the schema (eg. http://www.isotc211.org/2005/mdb/1.0/2013-06-24) could define the conversion to apply in CSW request:
```
<?xml version="1.0"?>
<csw:GetRecordById xmlns:csw="http://www.opengis.net/cat/csw/2.0.2"
  service="CSW"
  version="2.0.2"
  outputSchema="http://www.isotc211.org/2005/mdb/1.0/2013-06-24">
    <csw:Id>cecd1ebf-719e-4b1f-b6a7-86c17ed02c62</csw:Id>
    <csw:ElementSetName>brief</csw:ElementSetName>
</csw:GetRecordById>
```
For the time being the 'own' value is a workaround for that.