Geopackage Support

[ghost]

We should update this to reflect the status of what we need to do in order to ensure that this supports geopackage uploads in whatever way is proper

[ghost]

Should be in some form as of this GeoNode#32 but we need to ensure it does everything we need, which should hopefully be documented here.

(Boundless JIRA link that should point here, under the name of "Multilayer" support: https://issues.boundlessgeo.com:8443/browse/NODE-410)

[ghost]

The existing geopackage support is actually cool, but there were some interface issues that had to be ironed out which are included in #20 along with multifile changes.