Expand search capability to multiple standard_names or other terms.

[MathewBiddle]

As of right now we define a specific list of CF standard_names to search for. We also only allow a user to select one standard name. Would it be possible to add a multi-select here? Maybe add in the full CF standard names list?

https://github.com/oceanhackweek/ohw19-project-co_locators/blob/9e94f52eece49b62542116b3c4044ed00c29defc/colocate/ui.py#L101-L108

@michaelgmorley mentioned this would be a nice feature to add.

Maybe using something like this notebook, but expanding it's capabilities to be generic for all erddap's?