Initial implementation of turntable orbit method

[jmwright]

This is a targeted PR adapted from the turntable rotation implemented by @mbway in #220

This feature has been discussed in #353

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 22 lines in your changes missing coverage. Please review.

Project coverage is 87.58%. Comparing base (e720033) to head (617b531).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
cq_editor/widgets/occt_widget.py	33.33%	16 Missing and 2 partials ⚠️
cq_editor/preferences.py	0.00%	2 Missing ⚠️
cq_editor/widgets/viewer.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #525      +/-   ##
==========================================
- Coverage   88.40%   87.58%   -0.82%     
==========================================
  Files          20       20              
  Lines        1802     1828      +26     
  Branches      201      208       +7     
==========================================
+ Hits         1593     1601       +8     
- Misses        164      179      +15     
- Partials       45       48       +3     

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[jmwright]

CodeCov refuses to see the mouse movements that qtbot is injecting, which I guess is common. I am not going to delay the merge of this because of that.