Fully curate dependencies #1112
Description
The recent changes to remove unwanted dependencies has not gone well, e.g. #1111.
We need to fully curate all of the dependencies, for install, test and examples. This includes checking that we are not including a dependency that we don't use, and we are not excluding a dependency that we need. In some situations we may be relying on a dependency being present when we don't really need to.
The big one was dask leading to bokeh (#1104). Another that comes to mind is spatialpandas which used to be hard dependency and now isn't, but we still import it in core code and do not gracefully handle the situation if it is not installed.
This needs to be checked from both the pip and conda point of views via pyctdev, and ideally manually as well. All the CI tests are currently using conda so we should add at least one CI test run using pip on each of the 3 major platforms so that pip dependencies are covered by CI.