Introduce an event on_lees_edwards_change

[davidbbeyer]

Fixes #4874

Description of changes:

• introduced a new event function on_lees_edwards_change() to be triggered when the LE BC is changed
• the order of instantiation of LB and LEbc no longer matters: collision kernel is automatically updated

[RudolfWeeber]

LGTM. Could you please check the codecov tst for the patch (which shows that coverage deceased slightly) and make sure, we don't have important code untested?
Thanks!

[davidbbeyer]

As far as I can tell, it is only complaining about the function LBWalberla::on_lees_edwards_change() which I added. However, this function is (indirectly) tested in the additional test in /testsuite/python/lb_lees_edwards.py , so I guess it should not be a problem.

[jngrad]

Thanks for taking care of this! I have selected a few lines were we potentially have edge cases that may need extra attention.

[jngrad]

I noticed a lot of commits don't pass CI. Would you kindly update your virtual environment using our pep8 instructions?

[jngrad]

Thanks a lot!