Add Support for LayerGroups

[hblitza]

This adds a class LayerGroupClient and initiates it with a get and modify method.

@chrismayer please check

[chrismayer]

Thanks for this @hblitza. Looks very good. Please refer to the comments in the review and please also ensure that you add at least a test for each function.

[hblitza]

Thx for review!

[chrismayer]

Thanks for your ongoing work @hblitza. As stated in my review, please add at least one test for each function before we can merge this.

[hblitza]

Right, I will add a create layergroup method. while working on the tests, it became clear, we need one for testing...

[hblitza]

@chrismayer please check again.

[chrismayer]

Thanks for your ongoing work @hblitza. Looks good to me now. 2 minor comments you might want to have a look at before merging.

[hblitza]

Right, thanks!

[chrismayer]

Thanks for your ongoing work @hblitza. I am going to merge now.