Add point substruction, slightly improve tests

[pkulev]

Closes issue(s): [ ]

Description of the changes:

• What's new: Point substraction
• Fixes: add 'slow' marker to tests with sleep, use repr instead of magic method
• Refactoring:

Reviewers: [ @taptap ]

Task list:

• [x] submit pull request
• [x] review
• [x] testing
• [x] fixes after review, squashing, rebasing to master

[codecov-io]

Current coverage is 55.37% (diff: 100%)

Merging #46 into master will increase coverage by 0.07%

@@             master        #46   diff @@
==========================================
  Files            38         38          
  Lines          1727       1730     +3   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits            955        958     +3   
  Misses          772        772          
  Partials          0          0          

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