Add landfill endmember notebook

[jpolchlo]

Work in response to #70 has been done with respect to identifying endmembers in landfills that correspond to uncapped garbage. This work is in a development Jupyter notebook. This example needs to be clipped out and made standalone.

This will not close #70, since there is further work related to endmember selection that should be done.

[jpolchlo]

I have added the landfill endmember notebook as a Gist. This is not going to be committed to the repository for a few reasons. (1) After JPL republished AVIRIS to an HTTPS endpoint and retired the FTP, some of my code ceased to work, and rebuilding it would take time that doesn't feel worth spending. (2) The notebook itself is more of a demonstration of GeoPandas prowess than of any specific or interesting technique for HSI analysis. (3) Following the merge of #151, there will be a useful notebook demonstrating the endmember selection and unmixing techniques that the landfill notebook was intended to show.

As such, I'm closing this issue.