Fully resolving #623

[henrydingliu]

• Adding feature engineering to Development ML in order to allow the regression to accept extrapolated data points (i.e. valuation)
• Adding sample weights to DevelopmentML in order to allow dropping specific data points
• Adding various tests

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.82%. Comparing base (d4b2c42) to head (a219c24).
⚠️ Report is 4 commits behind head on BZ_feat_eng_w_drop.

Files with missing lines	Patch %	Lines
chainladder/development/learning.py	87.09%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           BZ_feat_eng_w_drop     #635      +/-   ##
======================================================
+ Coverage               84.44%   84.82%   +0.37%     
======================================================
  Files                      84       84              
  Lines                    4823     4850      +27     
  Branches                  610      618       +8     
======================================================
+ Hits                     4073     4114      +41     
+ Misses                    538      524      -14     
  Partials                  212      212              

Flag	Coverage Δ
unittests	84.82% <89.47%> (+0.37%)	⬆️

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