Create a "build an accessor" tutorial notebook

[stevehadd]

I demonstratined creating a minimal accessor to load a netcdf file in through a pyearthtools accessor. It would be good to have a basic tutorial on steps to go through to build an accessor and then potentially also to add it to a site archive for a very minimal set of netcdf files on disk.
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We could hten move on to an a tutorial on creating an accessor for a cloud data set.