Move tests to github actions; add optional amici dependency

[yannikschaelte]

• Move tests to github actions
• Add optional amici dependency
• Cache dependencies for faster runs
• Use pytest for tests (more flexible, has unittest as subset)
• Use codecov instead of coveralls because that was easier for me (coveralls should also be straightforward to set up: https://coveralls-python.readthedocs.io/en/latest/usage/configuration.html#github-actions-support)

Needs to be done on yaml2sbml-dev/yaml2sbml:

• Register at codecov
• Create Settings > Secret CODECOV_TOKEN
• Update the README.md badge sources

[jvanhoefer]

Looks cool! Thank you a lot!