Add arex/arey to available alignment errors

[roman-martin]

-add support for arex/arey aperture misalignments from MAD-X
-adapt find_element_ids() to find aperture marker more reliably
-checks if errors exist/are non-zero are now done before calling the respective add_xyz_error_to() functions, not in the functions themselves

Based on changes from PR #37

[aoeftiger]

I would opt for leaving the default values of dx=0, dy=0 and angle=0 (and accordingly add them for arex=0, arey=0) in the argument signatures to have an indication of what value type is requested for this argument.

I can live with removing return upon zero values inside the add_...() functions though, as long as no "empty" error elements are added in apply_madx_errors (as it is the case here, so that's fine with me).

--> @roman-martin do you want to re-add the default arguments?

[aoeftiger]

.. and potentially rebase this branch against #37 .

[aoeftiger]

@rdemaria @martinschwinzerl ready 😄