Arrow length

[jeffreyrosenbluth]

This should be ready to merge.

The headSize and tailSize fields have been dropped in favor
of headLength and tailLength, this has several advantages:

• more intuitive
• consistent with gaps
• allows lengths traversal which can be used e.g. to make the head and tail
  the same length so things like the uarrow example work.

We choose the names headLength and tailLength since they are
perpendicular to shaft width.

The missile head has been removed since it did not have a proper joint,
this would be visible for translucent heads, additionally it does not
allow for the calculation of radius given length. The user can always
achieve similar results by applying a cubic spline to dart (which is
what we were doing anyway).

Also the arrowhead... functions have been cleaned up and simplified
considerably.

[jeffreyrosenbluth]

I'd like to merge this in before 1.2 for two reasons:

• We lost the traversal that set both the head and tail length to
  the same value. This will restore it, and I feel that is a important
  use case, in particular with lineTail.
• We have a lot of breaking changes going into 1.2, I think it's a
  good idea to include as many as we know will about now so
  that it is possible for minor subsequent releases not to break things.

[byorgey]

Oh, good catch. However, it would be even better if we could replace these with something like lw thick or whatever looks right.

[jeffreyrosenbluth]

Should be good to go

[byorgey]

Looks great, thanks for putting this together @jeffreyrosenbluth !