fix:calculation of elements_added on join

[cunla]

Thanks for this library! It is very helpful.
I found a small bug in the CMS.join() - here is a fix and a test.

[barrust]

Great catch! The current failures are a codecov upload problem on their end.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6feb143) 99.61% compared to head (c94820b) 99.61%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #119   +/-   ##
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  Coverage   99.61%   99.61%           
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  Files          16       16           
  Lines        1801     1801           
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  Hits         1794     1794           
  Misses          7        7           

Files	Coverage Δ
probables/countminsketch/countminsketch.py	100.00% <100.00%> (ø)