Function getById

getById(layerId, thematicId): JLayerThematic
JMap.Layer.Thematic.getById

Returns a specific layer thematic.
Parameters
- layerId: JId
  The JMap layer id
- thematicId: JId
  The thematic id
Returns JLayerThematic
Throws
Error if no layer found for the id, if the layer is a layer group, or if the thematic doesn't exist.

Example
```
// returns thematic id=3 of layer id=4
JMap.Layer.Thematic.getById(4, 3)
```