Changing to boolean argument in parallel_assert

[brownbaerchen]

Since the regular Python assert works with booleans instead of callables, I was a bit confused why I had to add lambda for the parallel version now. The only situations where a callable is required as argument in parallel_assert that @connorjward could come up with are dubious coding practice.
So, to streamline the workflow for new users, we decided to switch to boolean argument, while still allowing callables, but raising a warning in that case.

I also added a basic CI pipeline with some unit tests for this. This includes tests that passing callables to parallel_assert still works.

[connorjward]

This is brilliant. Thank you!

[connorjward]

very very close now

[connorjward]

Thanks!