render diameter when a feature is selected

[k-yle]

If a node has a tag like diameter or radius, iD will now render a circle when you select the node.

• As per the wiki, spaces are allowed between the number and the unit.
• These tags are supported: diameter, diameter_crown, hole:diameter, radius, crown_radius, and seamark:anchor_berth:radius.
• Many different units of measurement are supported, including a combination of feet and inches
• The default unit is m for all tags except diameter, where the default unit is mm and seamark:anchor_berth:radius where the unit is defined by a separate tag (seamark:anchor_berth:units)

Outstanding issues:

• A separate PR will be required to add a field to the "Turning Circle" preset (and possibly the tree preset for diameter_crown?)
• There is a problem with the way selection.selectAll is used, which causes the rendering to stop after certain options like the Extract operation

[tordans]

I really like this idea. We are using diameter on trees a lot in Neukölln to render a more realistic Map like https://strassenraumkarte.osm-berlin.org/?map=micromap#19/52.47378/13.43892 so that will help at lot to get a feel for this value.