Fix AttributeError for misaligned non-multipoles

[roman-martin]

Fixes issue #34.

[roman-martin]

This an alternative solution to #36.
Here we keep the possibility to get AttributeErrors if the magnet error input does not fit into our model, instead of just ignoring the input.

[roman-martin]

I added a fix to #38, maybe Adrian should have a look first

[aoeftiger]

The following rebase of Roman's version fixes

• a missing element definition,
• an appropriate test_line.py update and
• the merge-conflicts when rebasing on the latest master branch state

https://github.com/aoeftiger/pysixtrack/tree/hotfix/ealign_non-multipole

--> if you base this PR on the above branch, Roman's commit contents are preserved and everything is good to go.

Summary:

This PR fixes a bug which occurs when trying to apply empty knl errors to non-multipole elements.
Besides this fix, the errors will be applied to the element_names corresponding element instead of the element instances directly -- this is the same behaviour as in MAD-X and avoids problems with / resolves issue #38 .

Tests run cleanly:

https://github.com/aoeftiger/pysixtrack/actions/runs/216470070

[aoeftiger]

@roman-martin has integrated my changes in his branch, should be good to go for the merge.