support gmx:anchor in iso19139 otherconstraints

[pvgenuchten]

INSPIRE recommends to use a gmx:anchor in otherconstraints, to be able to reference a value from https://inspire.ec.europa.eu/metadata-codelist/ConditionsApplyingToAccessAndUse

owslib currently supports characterstring only for this property

suggestion is to extend this to include support for gmx:anchor, the value captured should be the xlink:href attribute (unless None)

an example of use of anchor can be found at https://data.isric.org/geonetwork/srv/api/records/5cd5336c-2f45-4430-a9a8-312aa2095cb6/formatters/xml