Make ISO Category in Geonode 1-n according to ISO19115:2003

[kikislater]

Is your feature request related to a problem? Please describe. Cardinality about topic category in Geonode is 1-1 whereas in multiple platform which implement ISO Category such as GeoNetwork or ArcGIS Online implement 1-n cardinality allowing users to choose multiple categories.

Describe the solution you'd like

• Improve ISO Category in Geonode cardinality to 1-n
• Allow users to select multiple categories through WebGUI and API
• Additionally if set in Geonode it allows to add more categories selecting one which is present in ISO and one which doesn't exists so custom categories if choice is let to enable multiple doesn't break platform compatibility

Describe alternatives you've considered not considered

Additional context Selecting category is a drop down menu, it needs to be change as well to select multiple values

[kikislater]

@eblondel @juldebar : useful information to avoid bug using geonapi