[FIX] Outliers widget no longer checks classes and doesn't crash on singular covariances matrices

[janezd]

Issue

• Outliers widget does not work with multiple classes.
• ... and crashes when variance matrices is singular, instead of reporting an error.
• EllipticEnvelopeLearner removes instances without classes.

Fixes #2588.

Description of changes

• Remove tests for classes
• Fix the crash for singular data
• Set proper preprocessors for EllipticEnvelopeLearner

Tests do not add much new; they passed before, too. The PR actually removes tests that checked for what shouldn't had been checked.

Includes

• Code changes
• Tests

[codecov-io]

Codecov Report

Merging #2677 into master will decrease coverage by <.01%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master    #2677      +/-   ##
==========================================
- Coverage   75.65%   75.65%   -0.01%     
==========================================
  Files         333      333              
  Lines       58422    58418       -4     
==========================================
- Hits        44202    44197       -5     
- Misses      14220    14221       +1

[jerneju]

Where are these preprocessors used?

[kernc]

orange3/Orange/base.py

Lines 132 to 143 in 8ee65c0

	def preprocess(self, data):
	"""Apply the `preprocessors` to the data"""
	for pp in self.active_preprocessors:
	data = pp(data)
	return data
	@property
	def active_preprocessors(self):
	yield from self.preprocessors
	if (self.use_default_preprocessors and
	self.preprocessors is not type(self).preprocessors):
	yield from type(self).preprocessors