Edits to classification notebook

[tcmetzger]

As described in #475, this PR contains various edits to the classification example notebook.

Most notably, I have updated wording and structure to clarify the goal of the notebook and edited all text to conform to the Microsoft Writing Style Guide (https://docs.microsoft.com/en-us/style-guide/welcome/). I have also updated the filename according to the scheme proposed in #488.

cc @giancastro, @brendancol, @marcozimmermannpm

[tcmetzger]

@giancastro @brendancol Three ideas/requests:

• For the introductory paragraph, it would be good to briefly mention why xarray-spatial is a good tool to use for classification in this context. So far, I only have a sentence saying that xarray-spatial "contains different classification methods" - it would be nice to be a little more specific about why one would want to use xarray-spatial to classify data.
• I think it would be helpful to include some more detail about the basic pros and cons of each of the four classification methods. I have tried to add some background, but would certainly appreciate your input.
• I would like to see some kind of concluding paragraph after the last example in the notebook. I would suggest either a "Next steps" section that could suggest other data sets/areas of interest to try with the four classification methods - or a summary section that highlights once again the pros and cons of each of the four classification methods and links to some other, more advanced methods (Geometric Interval, Standard Deviation, etc.).

[brendancol]

@tcmetzger @giancastro this is looking great!

I a bit torn on the use of matplotlib, we can also use the Elevation cmap from datashader.colors and avoid the extra dependency. Let's go ahead and get MS PR going and we can switch that out later if needed