Added check if particles are not in ELC gap region

[biermanncarl]

Fixes Issue #4009

Description of changes:

• add a check in both positive and negative direction with error message

[jngrad]

please use maintainer/CI/fix_style.sh :-)

[biermanncarl]

please use maintainer/CI/fix_style.sh :-)

I tried, but doesn't work on my local machine properly yet ... It didn't do anything. Sorry.

[espresso-ci]

Your pull request does not meet our code formatting rules. Specifically, I suggest you make the following changes:

diff --git a/testsuite/python/elc.py b/testsuite/python/elc.py
index 24cb9c9e7..c1d9a9085 100644
--- a/testsuite/python/elc.py
+++ b/testsuite/python/elc.py
@@ -85,5 +85,6 @@ class ElcTest(ut.TestCase):
         with self.assertRaises(Exception):
             self.integrator.run(2)
 
+
 if __name__ == "__main__":
     ut.main()

To apply these changes, please do one of the following:

• You can download a patch with my suggested changes here, inspect it and make changes manually.
• You can directly apply it to your repository by running curl https://gitlab.icp.uni-stuttgart.de/espressomd/espresso/-/jobs/265635/artifacts/raw/style.patch | git apply -.
• You can run maintainer/CI/fix_style.sh to automatically fix your coding style. This is the same command that I have executed to generate the patch above, but it requires certain tools to be installed on your computer.

You can run gitlab-runner exec docker style afterwards to check if your changes worked out properly.

Please note that there are often multiple ways to correctly format code. As I am just a robot, I sometimes fail to identify the most aesthetically pleasing way. So please look over my suggested changes and adapt them where the style does not make sense.

[jngrad]

thanks!

[jngrad]

Could you please amend these two paragraphs to explain the method will throw if particles enter the gap?

espresso/doc/sphinx/electrostatics.rst

Lines 258 to 259 in 2518718

	particles' z-coordinate. When there is no empty slab of the specified size,
	the method will silently produce wrong results.

espresso/doc/sphinx/magnetostatics.rst

Lines 99 to 100 in 2518718

	particles' z-coordinate. When there is no empty slab of the specified size,
	the method will silently produce wrong results.

After that you can remove the WIP from the title, and we're good to merge.