Remove ThinElement (#53)

I think this implements @ax3l's suggestion in
#47 (comment),
though I'm not sure if it really addresses @wlin6927's original concern
raised in
#47 (comment)
(which I interpreted as something along the lines of "a `Kicker` element
could be either thick or thin", etc.).

---------

Co-authored-by: Axel Huebl <axel.huebl@plasma.ninja>

Author: EZoni

src/pals/kinds/Marker.py

@@ -1,13 +1,17 @@
 from typing import Literal
 
-from .mixin import ThinElement
+from .mixin import BaseElement
 
 
-class Marker(ThinElement):
+class Marker(BaseElement):
     """Zero length element to mark a particular position
 
     The main purpose of this thin element is to name a position in the beamline.
     """
 
     # Discriminator field
     kind: Literal["Marker"] = "Marker"
+
+    # Segment length in meters (m)
+    # Always 0, cannot be modified
+    length: Literal[0.0] = 0.0

src/pals/kinds/mixin/ThickElement.py

@@ -1,5 +1,5 @@
 from typing import Annotated, Literal
-from annotated_types import Gt
+from annotated_types import Ge
 
 from . import BaseElement
 
@@ -11,4 +11,4 @@ class ThickElement(BaseElement):
     kind: Literal["ThickElement"] = "ThickElement"
 
     # Segment length in meters (m)
-    length: Annotated[float, Gt(0)]
+    length: Annotated[float, Ge(0)]

src/pals/kinds/mixin/ThinElement.py

@@ -1,3 +1,2 @@
 from .BaseElement import BaseElement  # noqa: F401
 from .ThickElement import ThickElement  # noqa: F401
-from .ThinElement import ThinElement  # noqa: F401