Function getSelfOrChildren

getSelfOrChildren(layerId): JLayer[]
JMap.Layer.getSelfOrChildren

If the layerId is a layer returns it. If the layerId is a layer group, returns all of its layers children (remove all layer groups).

Returns an empty array if it's an empty layer group
Parameters
- layerId: JId
  The JMap layer id
Returns JLayer[]
Throws
Error if no layer found for the id

Example
```
// returns an array with only JMap layers (no layer group)
JMap.Layer.getSelfOrChildren(3)
```