Bugfix: add/remove layer to layergroup

[Kilometerfresserin]

Bug description:

• The Geoserver API returns lists for styles & publishables in a layergroup in case the layergroup contains more than one layer; if the layergroup contains only one layer, however, "styles" may either be a string or a dictionary.
• The current implementation of add_layer_to_layergroup would however only check if styles is a string and thus fail when confronted with a dictionary.

Solution description:

• The "single-layer" check now checks for "is not a list" rather than for "is a string"