ClipPlanesEvaluator cleanup

[atomicsulfate]

• Extract some functions out of evaluator class into a private namespace.
• Small documentation changes.
• Minor code rearrangements.

No behaviour changes.

[codecov]

Codecov Report

Merging #2244 (f56af67) into master (6ce91d1) will increase coverage by 0.27%.
The diff coverage is 82.97%.

@@            Coverage Diff             @@
##           master    #2244      +/-   ##
==========================================
+ Coverage   67.40%   67.67%   +0.27%     
==========================================
  Files         312      312              
  Lines       27670    27660      -10     
  Branches     6182     6188       +6     
==========================================
+ Hits        18650    18718      +68     
+ Misses       9020     8942      -78     

Impacted Files	Coverage Δ
@here/harp-mapview/lib/ClipPlanesEvaluator.ts	53.36% <82.97%> (-5.08%)	⬇️
@here/harp-mapview/lib/VisibleTileSet.ts	78.25% <0.00%> (-1.57%)	⬇️
@here/harp-mapview/lib/MapMaterialAdapter.ts	92.14% <0.00%> (-0.72%)	⬇️
@here/harp-mapview/lib/PickListener.ts	100.00% <0.00%> (ø)
@here/harp-mapview/lib/poi/PoiManager.ts	12.85% <0.00%> (ø)
@here/harp-vectortile-datasource/lib/OmvUtils.ts	90.56% <0.00%> (ø)
@here/harp-mapview/lib/text/TextElementBuilder.ts	100.00% <0.00%> (ø)
...here/harp-mapview/lib/text/TextElementsRenderer.ts	72.11% <0.00%> (ø)
...e/harp-mapview/lib/geometry/TileGeometryCreator.ts	72.68% <0.00%> (ø)
@here/harp-mapview/lib/text/TextElement.ts	93.75% <0.00%> (+0.07%)	⬆️
... and 3 more

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