Change the coveralls github action to fix missing coveralls in PRs

[nils-braun]

I moved from coveralls to codecov, as it has better support for forked branches (to help solving #732) and because it has a nicer UI ;-)

[github-actions]

Result of Benchmark Tests

Benchmark	Min	Max	Mean	Mean on Repo HEAD
tests/benchmark.py::test_benchmark_small_data	11.45	12.53	12.05 +- 0.55	12.66 +- 0.25
tests/benchmark.py::test_benchmark_large_data	6.12	6.22	6.17 +- 0.04	6.15 +- 0.08
tests/benchmark.py::test_benchmark_with_selection	8.16	8.25	8.20 +- 0.04	8.45 +- 0.07

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@535de97). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #734   +/-   ##
=======================================
  Coverage        ?   96.28%           
=======================================
  Files           ?       18           
  Lines           ?     1695           
  Branches        ?      322           
=======================================
  Hits            ?     1632           
  Misses          ?       31           
  Partials        ?       32           

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