Fix visualization of arrows with zero shaft length.

[tkoolen]

Avoid the inverse, which was causing a SingularException when the shaft length was computed to be zero.

Basically, turn

vis -> shaft -> head

into

vis -> shaft
    -> head

where only the translation and rotation part of the old shaft transform is now applied to vis.

I've verified that the animation interpolation still looks fine.

Fixes #105.

[codecov-io]

Codecov Report

Merging #107 into master will increase coverage by 0.2%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master    #107     +/-   ##
========================================
+ Coverage    75.2%   75.4%   +0.2%     
========================================
  Files          13      13             
  Lines         363     366      +3     
========================================
+ Hits          273     276      +3     
  Misses         90      90

Impacted Files	Coverage Δ
src/arrow_visualizer.jl	100% <100%> (ø)	⬆️

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[tkoolen]

I hadn't noticed #106, but I think this is a better solution anyway.

[rdeits]

Yup, agreed. Thanks!