Feature/longitudinal exciter with arbitrary digital waveforms

[paa27]

Description

Adds a LongitudinalExciter element to xtrack, which can play an arbitrary time-dependent voltage function. It is the longitudinal analogue of the transverse Exciter.

This element allows to simulate complicated time-dependent RF programs (e.g. band-limited noise) by inputing a digital waveform.

Checklist

Mandatory:

• I have added tests to cover my changes: test_longitudinal_exciter.py + added element to test_elements.py
• [~] All the tests are passing, including my new ones: only checked the tests I added.
• 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