Prepare for v3.0.0

[bdestombe]

• Closes Deprecated kwargs were not removed in v2. #185. The x locations along the x-dimension where there is something, e.g. a connector, that introduces asymmetrical attenuation
• Closes Inconsistent ds.sections behavior #138. Inconsistent ds.sections behavior.
• Closes Only first level headers of example notebooks should be included in docs #176. Fixes headers in example notebooks
• Closes stokes variance; fit through zero? #134. Stokes variance estimation fits through zero by default.
• Closes Simplifying datastore.py #158. Simplifying datastore.py

[BSchilperoort]

Thanks for taking up all these issues! Nice work. I left some comments.

I do think that the datastore.py file can still be simplified more, ideally the max number of lines in a file is 1000. So perhaps not close that issue just yet.