Fix title and number of top species in samples composition plot

[AgnesBaud]

Description

Fix bug in title where the number for the top was actually the number of samples

• only plot taxa that are actually present, no matter if the number of taxa asked for is superior to the number of taxa present.

Definition of Done

• [ ] New code is tested with unit tests
• [ ] Documentation has been updated
• [ ] Pull Request has been revised by a maintainer of the project

[codecov-commenter]

Codecov Report

Merging #78 (0ab73f6) into master (6af855c) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #78   +/-   ##
=======================================
  Coverage   58.54%   58.54%           
=======================================
  Files          60       60           
  Lines        2458     2458           
=======================================
  Hits         1439     1439           
  Misses       1019     1019           

Flag	Coverage Δ
unittests	58.54% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
moonstone/plot/counts.py	61.11% <ø> (ø)

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