Add watershed information table

[corviday]

This PR addresses a UI issue with the new streamflow portal. The streamflow portal has the user click on a single point on the map (streamflow does not make sense averaged across an area) but it can be difficult for the user to tell whether they have indeed clicked on the point they're interested in; the UI doesn't offer any feedback. These changes add a table showing some information about the watershed that drains to the point the user clicked on: the latitude and longitude of the selected point (from the map) and information provided by the watershed API: the area of the watershed that drains to that point, highest and lowest elevations, and the Melton ratio.

Changes:

• Adds AttributeValueTable, a table that will display a collection of objects with attribute, value and units members
• Adds WatershedSummaryTable, a component that queries the watershed backend for data and passes it to an AtributeValueTable
• getWatershed() and getWatershedGeographyName() in ce-backend.js for querying the watershed API
• validateWatershedData() and parseWatershedTableData() in util.js for checking the API response and formatting it into attribute/value/unit objects for the table
• tests for the new stuff, which I think is more linecount than the new code

Demo.

Resolves #405

[rod-glover]

Nice job. Great testing. (Isn't it amazing how more than half the effort of development is in creating tests?)

A couple of thoughts about the UI:

• Is it possible to change the hint popup on the circle marker in the map toolbar to read something like "Select a point on the map"? I recall looking into this for some other map tool, but can't remember if it proved intransigent. Alternatively, change the prompt in the unselected state to read "Select a point on the map, using the circle marker tool, to see watershed information."
• The watershed information is great. May I suggest making it more explicit that the watershed in question is the one draining to the selected point? So maybe a change to the heading of the table, and/or revising the label of the Area and Melton Ratio params to be explicit about this?

[rod-glover]

Totally minor: You can fold this into the class defn above: export default class ....

[rod-glover]

Do you feel like trying this? Only if you have time.

[corviday]

These are both great UI suggestions, thanks!

* Is it possible to change the hint popup on the circle marker in the map toolbar to read something like "Select a point on the map"? I recall looking into this for some other map tool, but can't remember if it proved intransigent. Alternatively, change the prompt in the unselected state to read "Select a point on the map, using the circle marker tool, to see watershed information."

It definitely seems like it should be possible to set a custom tooltip - "Draw a circlemarker" isn't very intuitive - but my efforts have come to naught. I updated the table text instead, as you suggested.

* The watershed information is great. May I suggest making it more explicit that the watershed in question is the one draining to the selected point? So maybe a change to the heading of the table, and/or revising the label of the Area and Melton Ratio params to be explicit about this?

Done, thanks!