Add 2D space charge element

cheetah/__init__.py

@@ -16,7 +16,8 @@
     Segment,
     Sextupole,
     Solenoid,
-    SpaceChargeKick,
+    SpaceChargeKick3D,
+    SpaceChargeKick2D,
     TransverseDeflectingCavity,
     Undulator,
     VerticalCorrector,

cheetah/accelerator/__init__.py

@@ -13,7 +13,8 @@
 from .segment import Segment  # noqa: F401
 from .sextupole import Sextupole  # noqa: F401
 from .solenoid import Solenoid  # noqa: F401
-from .space_charge_kick import SpaceChargeKick  # noqa: F401
+from .space_charge_kick import SpaceChargeKick2D  # noqa: F401
+from .space_charge_kick import SpaceChargeKick3D  # noqa: F401
 from .transverse_deflecting_cavity import TransverseDeflectingCavity  # noqa: F401
 from .undulator import Undulator  # noqa: F401
 from .vertical_corrector import VerticalCorrector  # noqa: F401