Add update / destroy API for VisComponent

[waxlamp]

TODO:

• [x] make GeoDots (and other GeoJS based components) use update API to fix "reset-to-0,0" bug
• [x] write documentation for new API methods

This PR adds update() and destroy() methods to VisComponent, allowing for in-place updates and cleanup upon destroying a component. These should make component lifecycles more explicit and manageable in automated contexts such as Resonant Lab and the Candela plugin for Girder.

Note that this now completes a sort of "CRUD" model for VisComponent: create == constructor(), read == render(), update == update(), and delete == destroy(). In particular, this means that render() should really just take the state of a component and make it visible - nothing more or less than that. Whether this is a valid model we want to officially promote for components is something that remains to discuss.

[codecov-io]

Codecov Report

Merging #495 into master will increase coverage by 2.9%. The diff coverage is 28.57%.

@@            Coverage Diff            @@
##           master     #495     +/-   ##
=========================================
+ Coverage   46.24%   49.14%   +2.9%     
=========================================
  Files          39       39             
  Lines        1544     1583     +39     
=========================================
+ Hits          714      778     +64     
+ Misses        830      805     -25

Impacted Files	Coverage Δ
components/SentenTree/index.js	40% <0%> (-5.46%)	:arrow_down:
components/SimilarityGraph/index.js	8.77% <0%> (ø)	:arrow_up:
components/Geo/index.js	18.18% <0%> (-1.82%)	:arrow_down:
VisComponent/mixin/VegaChart.js	83.87% <100%> (+61.64%)	:arrow_up:
components/TreeHeatmap/index.js	83.98% <100%> (ø)	:arrow_up:
components/GeoDots/index.js	11.94% <6.38%> (-1.4%)	:arrow_down:
VisComponent/index.js	60% <66.66%> (+2.85%)	:arrow_up:
util/vega/index.js	87.44% <0%> (+16.88%)	:arrow_up:

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[waxlamp]

@jeffbaumes this is ready for review.