fix:replace_spacecharge_with_PIC can now create space charge elements with apply_z_kick set to True

[MarkusSQ]

Description

fix:replace_spacecharge_with_PIC now creates PICCollection with longitudinal kick by default for FFTSolver3D

Added apply_z_kick parameter to replace_spacecharge_with_PIC. Added logic so that if solver is FFTSolver3D and no apply_z_kick parameter value is specified when calling replace_spacecharge_with_PIC, apply_z_kick is set to True when creating PICCollection.

Previously, PICCollection and therefore the line object's SpaceCharge3D elements would always be created with apply_z_kick as False when calling replace_spacecharge_with_PIC even with solver=FFTSolver3D. In other words, no longitudinal kick would be deposited regardless of specified solver. This change aims to fix that.

If the user really wants to use solver=FFTSolver3D with apply_z_kick=False then that is possible by explicitly specifying apply_z_kick=False when calling replace_spacecharge_with_PIC.

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]

Thanks for the bug fix with a test! Looks good to me