Have both field coverage diff and field coverage

[manycoding]

Addressing my concerns in #46 , now we have both coverage and fields with >5% difference in coverage.

import arche
arche.rules.coverage.get_difference(g.source_items.job, g.target_items.job).show()

That's coverage. I don't know about the correct name so I made it as precise as possible - we get this coverage from dividing job stats on total items.

And that's the >5% difference (from substraction of values from the previous graph)

That's how it looks with % tickformat, which is an option, not included in this pr

That's how it looks on unbalanced data (>100% coverage)

[codecov]

Codecov Report

Merging #51 into result_stats will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@               Coverage Diff                @@
##           result_stats      #51      +/-   ##
================================================
+ Coverage          62.2%   62.22%   +0.01%     
================================================
  Files                25       25              
  Lines              1585     1591       +6     
  Branches            278      280       +2     
================================================
+ Hits                986      990       +4     
- Misses              573      575       +2     
  Partials             26       26

Impacted Files	Coverage Δ
src/arche/report.py	67.3% <0%> (-1.32%)	⬇️
src/arche/rules/coverage.py	100% <100%> (ø)	⬆️

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