Open dataset wrapper

[JoelLucaAdams]

Add a lightweight wrapper for open_dataset that simply calls xarray.open_dataset. This is so that the user only needs to import sdf_xarray and makes it slightly easier for new users to work with the code. Additionally added tests and updated docs

Some other minor improvements made in the same branch:

• Add missing drop_variables from open_datatree
• Docstring fixes and addition of information on variables
• Some minor docs cleanup
• Add padding to bottom of jupyter cell outputs to visually separate the xarray output from the following text

Resolves #92

#93 merged Wait for #93 to be merged so that the load_deck parameter can be included in the open_dataset wrapper

[LiamPattinson]

There are a lot of duplicate tests here. Could you instead use parameterise the tests over xr and sdfxr?

[JoelLucaAdams]

Good point... since this would be quite a large refactor of the tests I'll do it in a separate branch. See #100

[LiamPattinson]

If the test updates will wait for a later PR, I think this should be good.