Add function to sketch arc given end point and radius

[spike77453]

This adds a function to sketch an arc given only the end point and the arc's radius.

The start point of the arc is assumed to be self._endPoint() of the sketch. A third point on the arc (the arc's midpoint) is constructed by creating a vector rectangular to the arc's cord with magnitude equal to the sagitta of the arc, added to the midpoint of the arc's cord (see https://math.stackexchange.com/a/3482681).

Edge.makeThreePointArc is used to create the arc with these three point.

[codecov]

Codecov Report

Attention: Patch coverage is 30.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 95.19%. Comparing base (a175cb8) to head (68e08fa).

Files with missing lines	Patch %	Lines
cadquery/sketch.py	30.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1705      +/-   ##
==========================================
- Coverage   95.29%   95.19%   -0.10%     
==========================================
  Files          27       27              
  Lines        6712     6721       +9     
  Branches     1001     1001              
==========================================
+ Hits         6396     6398       +2     
- Misses        193      200       +7     
  Partials      123      123              

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