Remove confusion between cartesian and spherical coordinates for ShellEnvelope and BoxEnvelope

[JulienPeloton]

This PR is linked to the discussion in #42.
Main changes:

• Specify the coordinate system when instantiating a ShellEnvelope with (x, y, z)
• BoxEnvelope accepts only Point3D with cartesian coordinates (instance + intersection).
• Rename sphericalToEuclidean to sphericalToCartesian, euclideanToSpherical to cartesianToSpherical in Utils.scala.

A few other fix and change made + unit tests added.

[codecov-io]

Codecov Report

Merging #44 into master will increase coverage by 0.3%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master     #44     +/-   ##
========================================
+ Coverage   98.99%   99.3%   +0.3%     
========================================
  Files          18      18             
  Lines         697     716     +19     
  Branches      166     192     +26     
========================================
+ Hits          690     711     +21     
+ Misses          7       5      -2

Impacted Files	Coverage Δ
src/main/scala/com/spark3d/utils/Utils.scala	100% <ø> (ø)	⬆️
...la/com/spark3d/geometryObjects/ShellEnvelope.scala	97.53% <100%> (+0.09%)	⬆️
...in/scala/com/spark3d/geometryObjects/Point3D.scala	100% <100%> (ø)	⬆️
...cala/com/spark3d/geometryObjects/BoxEnvelope.scala	100% <100%> (ø)	⬆️
...ain/scala/com/spark3d/spatial3DRDD/SphereRDD.scala	100% <100%> (+5%)	⬆️
...com/spark3d/spatialOperator/CenterCrossMatch.scala	100% <0%> (+2.5%)	⬆️

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