ENH: Increase test coverage #18
Description
Is your feature request related to a problem?
We have not been systematic about test coverage. A first glance (e.g., at main.py) shows that we are missing many obvious cases (like a test running main providing neither main_target nor main_targets.
Describe the solution you would like to see
Go through the coverage reports at https://app.codecov.io/github/ttsim-dev/ttsim and pick all low-hanging fruits.