Schottky monitor

[ChristopheLannoy]

Description

Adding Schottky monitor element, example script and tests.

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

[giadarol]

Thank you!
Could you transform the notebook into a script. We avoid keeping notebooks in the xsuite repositories