Add support for the 'reference' attribute on the vector gm

[scottwittenburg]

The 'reference' attribute has been available to read/write on
the vector graphics method, but until now has not been implemented.
Now, if it is set, the default behavior of choosing a sensible value
for the arrow length based on the screen space availabe, text size,
etc., will be overridden. Instead the arrow length will simply be
chosen to match the reference attribute. Hence, this feature should
be used with care, as it could result in a vector legend with either
very large or very small reference arrow.

[scottwittenburg]

This addresses part of #386, by supporting the reference attribute of the vector graphics method.

Needs baselines from here.

[scottwittenburg]

@danlipsa Can you take a look at this when you get a chance? My goal was to keep your code implementing the default behavior intact (choosing an appropriate size for the vector legend arrow), while also supporting the reference attribute as an override.

ping @aashish24

[coveralls]

Coverage increased (+0.01%) to 70.903% when pulling 48420d1 on support-vector-reference-attribute into 22418d8 on master.

[danlipsa]

+1. Looks good!

[doutriaux1]

@scottwittenburg shouldn't we update the doc string to explain better what the NNormalize options really mean?

[scottwittenburg]

@doutriaux1 We definitely should, I was hoping that @aashish24 could eventually chime in since he wrote that code. And the commit message you pointed me to last week (or was it the one before) didn't make sense to me.

[doutriaux1]

@danlipsa i limites this pr to mac py3 let's use it to test your work. I tried to up xcode but no luck on that end either

[doutriaux1]

@danlipsa. I would say ssh into this and try to do your manual build here against the python that was pulled. What do you think?

[doutriaux1]

@danlipsa ok so py36 works but not py37 so i think you'll need to manually build for py37. Sorry about that.