Filter doesn't work for layer groups

[paul-dorsetcouncil]

Describe the bug
Dorset Council have recently started to use layer groups to handle layers with multiple geometry types (i.e. point, line and/or polygon). This excludes multi so polygon and multi-polygon would be a single layer of the geometry type multi-polygon. The layer groups will render and query correctly but fail for filtering.

The OGC standards do not include a definition for layer groups specifically however the specification for WMS does permit nested layers within a capability statement. The key reason this is failing for filtering is that layer groups are not included in either the WFS or CSW capabilities because they are not directly usable via these services. This is logical because traditionally the application will add a feature service of a particular geometry type (say point or line) and so you need to add the group members individually.

Different mapping servers will operate in a different ways with some only permitting a single layer to be exposed and others, such as GeoServer, allowing the nested layers via "Named tree". However the point above would likely still apply to the other services (WFS/CSW) and only methods that expose the group members via all three services would work for filtering in any case.

We need to decide on a suitable method to handle layer groups.

To Reproduce
Steps to reproduce the behaviour:

1. Add a layer group to your map.
2. Filtering will not work.

Expected behaviour
Ideally we would handle layer groups so that admins are made aware of what filtering options are available based on the capabilities when using the management area. The best method is probably to scan the capabilities on save and adjust the options available depending on which services are available (WFS / WMS / CSW). This way the system should be able to tell the user whether or not filtering will be available.

Additional context
The design of this functionality is being done with mind to the design of the layer tool in the Dorset Council QGIS plugin to provide as much consistency as possible across the two platforms. This will hopefully lead to better user outcomes as users will be able to see layers presented in a similar way in both applications (i.e. a single layer or a group with sublayers etc).