Update quiver trace API

[emilykl]

Closes#7944

• rename sizeref attribute to lengthfactor (since sizeref for other traces works differently, as a divisor rather than a multiplier, we shouldn't use the same name here)
• rename sizemode to lengthmode (to parallel lengthfactor and because it's really about vector length rather than size)
  • accepted values 'scaled' and 'raw' remain the same
• rename anglemode to arrowref
  • accepted values 'data' and 'paper' remain the same

Also:

• Fix incorrect default for arrowref (was 'axis' which is not an accepted value; changed to 'data')
• Improve lengthmode: 'scaled' arrow length calculation
• Update Jasmine tests and image tests accordingly
• Update some other attribute descriptions

[camdecoster]

I left you a few suggestions. I like the new attribute names.

[camdecoster]

Can this comment be addressed?

[emilykl]

Addressed by 1315d07. I updated the point distance calculation to be more accurate, and as a bonus now it covers the case where all the points are in a horizontal or vertical line.

There's nothing we can do about the case where all points have the same x or y value; I think it's reasonable to just default to 1 in that case.

[camdecoster]

arrowref is also a good choice.

[camdecoster]

👏

[emilykl]

arrowref is also a good choice.

Glad you're OK with it. It occurred to me that in the future we may want to add the option to specify vectors by angle and magnitude, rather than u/v, so arrowref seemed more appropriate.