Enable csv-zip download of plot obs and related data

[regetz]

For any given set of plot observations that can be requested using the GET /plot-observations endpoint and its variants (e.g., GET /projects/pj.123/plot-observations to get plot observations for project pj.123), clients should have the ability to request an entire bundle of data that includes not only the plot observation records themselves, but related data about taxa, communities, project and contributing party details, etc.