Only keep relevant files for npm module/package

[NatsuDzn]

• I'm submitting a ...

  • bug report
  • feature request
  • support request => Please do not submit support request here, see note at the top of this template.

• What is the current behavior?

Some files are still present in the npm modules, however we don't need them
That makes the package larger for no real reason

• What is the expected behavior?

NPM modules should only have needed files

• What is the motivation / use case for changing the behavior?

Free more space

• Possible fixes

Add irrelevant files to https://github.com/Dessia-tech/plot_data/blob/master/.npmignore

[NatsuDzn]

I'm reopening this issue, because there is still some leftover in the package.
.nyc_output and coverage folder