Introduce a CircleCI step for Codecov

[Dzol]

Introduce the Codecov tool for coverage information

[coveralls]

Pull Request Test Coverage Report for Build 45

• 0 of 0 (NaN%) changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 42:	0.0%
Covered Lines:	24
Relevant Lines:	24

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💛 - Coveralls

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@3fa33ef). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             master     #6   +/-   ##
=======================================
  Coverage          ?   100%           
=======================================
  Files             ?      2           
  Lines             ?     24           
  Branches          ?      0           
=======================================
  Hits              ?     24           
  Misses            ?      0           
  Partials          ?      0

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Continue to review full report at Codecov.

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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