Make improvements to elm examples (with datashader, bokeh-geo, etc)

[PeterDSteinberg]

It would be nice to make use of the example datasets of datashader and its system for downloading those. Some examples with elm could benefit from using the U.S. census data and other data sets. We may also be able to leverage bokeh-geo, either by replacing it with an example, or using it in conjunction with elm.

TODO:

• Document how to get the datashader example data and its relevance to elm - both the relevance of datashader itself and the example data
• Documentation should show how to download that data in a way that the notebooks in ./examples of elm can find the datashader example data at a typical place or check several places or use an env variable to find it
• Document / show off bokeh-geo examples in a notebook - this could go into either datashader or elm.

[gbrener]

Ok, I assume that we'll want to add the documentation to https://github.com/ContinuumIO/elm/blob/master/docs/source/examples.rst? Marking as "ready" to work on.

[PeterDSteinberg]

examples.rst sounds like a good place for the new docs, @gbrener .

[PeterDSteinberg]

Related to this issue, I need to finish and merge #166 when datashader regridding /resampling options are merged. That notebook in #166 was held up by some unclarity / deficiencies I found in the scoring of supervised models.

[gbrener]

@PeterDSteinberg , PR holoviz/datashader#375 is now merged, however after a conversation with @jbednar we may want to be careful with the cvs.raster() API with respect to the upsample_method and downsample_method keyword arguments. These are very likely to change within the next day or two, including the philosophy of how these parameters are specified (the alternative will likely involve a settings-style object based on the param library, but it hasn't been decided yet).

[gbrener]

@PeterDSteinberg I marked the first two items as "done" now that we have a README and script in elm/examples directory with PR #166 .

[gbrener]

Closing issue, since all 3 boxes are checked.