add --coverage option

[HadrienG]

Relevant issue: #61

Also fixes #64

[codecov]

Codecov Report

Merging #65 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #65   +/-   ##
=======================================
  Coverage   90.21%   90.21%           
=======================================
  Files          12       12           
  Lines         736      736           
=======================================
  Hits          664      664           
  Misses         72       72

Impacted Files	Coverage Δ
iss/abundance.py	87.95% <ø> (ø)	⬆️

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