Add exit aperture after slicing.

[freddieknets]

Description

When slicing, apertures are automatically added between the slices if the element has name_associated_aperture set. However, no aperture is set after the last slice (which can be problematic when backtracking e.g. during aperture interpolation). This PR adds a final aperture at the end (before the exit_marker if present).

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

Optional:

• The code I wrote follows good style practices (see PEP 8 and PEP 20).
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts