Added methods to update and delete layergroups

[jeafreezy]

The following were implemented in this PR

• Reverted back to previous service_url as mentioned here since it's breaking some codes.
• Implemented layergroup delete
• Implemented layergroup update
  • Geoserver does not support update on layergroup name and workspace. So I could only implement update to support for the title, abstract text, metadata and keywords.
  • I actually tried updating a layer, it worked. But it removed previous layers from the layergroup. So I removed layer updating from the update_layergroup method. I will create another function to add layers to layer group as suggested here.
• I also added some checks e.g to confirm if a layer is valid in Geoserver and if a workspace is valid as well. I also validated if a layergroup is valid during layergroup update and deletion.
• Some other minor bug fixes.
• TODO: Tests for these methods and the add_layer_to_layergroup method.
  I will work on these in the next PR and I once the add_layer_to_layergroup method is implemented, we can close that issue.

[iamtekson]

Thank you very much @jeafreezy. It is really a cool feature with geoserver-rest. I will review this PR this week.

[iamtekson]

Thank you very much @jeafreezy for this wonderful PR. Everything looks good to me.

I am really sorry for taking so long time to review this PR.

[jeafreezy]

Thank you @iamtekson ! I haven't opened a PR for the adding a layer to layergroup function because I have had no luck doing so. I think it's not possible currently with Geoserver via REST. I will keep exploring anyways.

[iamtekson]

Yes, I think so. Either this functionality is missing from their documentation or it is not available till now. Hope we will find it one day.