Add multipolar components to solenoid

[szymonlopaciuk]

Description

• Add knl, ksl and related fields to the solenoid element to implement multipolar components.
• Also a small amount of refactoring in xtrack/beam_elements/slice_elements.py.
• Also see a comment below.

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

[szymonlopaciuk]

Also: does this do the right thing? If XTRACK_SOLENOID_NO_SYNRAD is defined then the variables are not defined and the compilation will fail. Also these values are passed as pointers to Solenoid_thick_track_single_particle but their updated values were not used (now they are though -- is that the correct way?).