[C029] Check if selected layer is a layergroup

[Gnafu]

Check for tag in the capabilities to know if the layer is a single layer or a group.
Trying to download from a group layer will result in a failure.

At first sight, there is no way (with the current OpenLayers implementation) to distinguish between a single Layer with an empty Keyword list ( tag) and a Layergroup (no tag at all)