Add regressor attribute into sklearn models

[GoshaLetov]

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Proposed Changes

Added self.regressor in SklearnPerSegmentModel and SklearnMultiSegmentModel

Closing issues

closes #76

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4a476f0) 89.29% compared to head (515b4f2) 89.29%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #199   +/-   ##
=======================================
  Coverage   89.29%   89.29%           
=======================================
  Files         198      198           
  Lines       12866    12868    +2     
=======================================
+ Hits        11489    11491    +2     
  Misses       1377     1377           

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[d-a-bunin]

I'll merge it after all tests pass.