query masked by WKT area

[7yl4r]

A common task is how to fetch data masked by an arbitrary region of interest (RoI) polygon.
Ideally the OPeNDAP standard & ERDDAP might allow for submission of a WKT, but masking could also be handled on the client side.

@ocefpaf : what are the first steps towards implementing RoI masking in erddapy?

Additional thoughts:

1. Should efforts to implement RoI queries be focused on the server or is this client workaround worthwhile?
2. Is this feature creep? Perhaps masking is a secondary task which should be kept separate from the core functionality of the API.

[ocefpaf]

We need to implement the griddap URL parser first. Working on that. Let's keep this one open until then. Once we have that using any object that has a _geo_interface_ should be possible.

[MathewBiddle]

Possibly a server side function in the future.
xref: ERDDAP/erddap#55

[ocefpaf]

Based on ERDDAP/erddap#55 looks like a server side function won't happen. However, we can do better with clients and I added an example in #246 to illustrate that. While it is not exactly a server-side precise shape query, it is a minimum data download with later trimming to be match the exact shape.

Closing this for now b/c I think this is the best we can do. Unless someone has a better idea for a PR.