Marker: Finalize (pals-project#42)

Remove the construction notice from `Marker`.

- improve docstring
- introduce `ThinElement` helper class

See pals-project/pals#98

Author: ax3l

src/pals/kinds/Marker.py

@@ -1,12 +1,13 @@
 from typing import Literal
 
-from .mixin import BaseElement
-from .utils import under_construction
+from .mixin import ThinElement
 
 
-@under_construction("Marker")
-class Marker(BaseElement):
-    """Zero length element to mark a particular position"""
+class Marker(ThinElement):
+    """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"

src/pals/kinds/mixin/ThinElement.py

@@ -0,0 +1,14 @@
+from typing import Literal
+
+from . import BaseElement
+
+
+class ThinElement(BaseElement):
+    """A thin base element with zero segment length"""
+
+    # Discriminator field
+    kind: Literal["ThinElement"] = "ThinElement"
+
+    # Segment length in meters (m)
+    # always 0 for thin elements, not modifiable
+    length: Literal[0.0] = 0.0

src/pals/kinds/mixin/__init__.py

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