[FIX] Feature Constructor cast function is not picklable

[ales-erjavec]

Issue

Feature Constructor can construct compute_value which are not picklable (defined in local function scope):

• orange3/Orange/widgets/data/owfeatureconstructor.py

  Lines 1032 to 1033 in cd3dfee

  	def cast(x): # pylint: disable=function-redefined
  	return values.get(x, nan)

• orange3/Orange/widgets/data/owfeatureconstructor.py

  Lines 1042 to 1047 in cd3dfee

  	def cast(e): # pylint: disable=function-redefined
  	if isinstance(e, (int, float)):
  	return e
  	if e == "" or e is None:
  	return np.nan
  	return parse(e)

  )

Fixes gh-5918

Description of changes

• Make the cast function passed to FeatureFunc picklable.
• Some refactoring of classes from oweditdomain.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #6002 (e33b38f) into master (04c0e73) will increase coverage by 0.03%.
The diff coverage is 96.42%.

@@            Coverage Diff             @@
##           master    #6002      +/-   ##
==========================================
+ Coverage   86.59%   86.63%   +0.03%     
==========================================
  Files         315      315              
  Lines       67317    67560     +243     
==========================================
+ Hits        58295    58531     +236     
- Misses       9022     9029       +7