Update Axis to a standard class

- Do not use dataclass - for consistency we prefer attrs.
- Remove ABC.meta - this is not an abstract base class.

python-spec/src/somacore/coordinates.py

@@ -2,15 +2,13 @@
 
 import abc
 import collections.abc
-from dataclasses import dataclass
 from typing import Any, Optional, Sequence, Tuple, Union
 
 import numpy as np
 import numpy.typing as npt
 
 
-@dataclass
-class Axis(metaclass=abc.ABCMeta):
+class Axis:
     """A description of an axis of a coordinate system
 
     Args:
@@ -21,9 +19,32 @@ class Axis(metaclass=abc.ABCMeta):
     Lifecycle: experimental
     """
 
-    name: str
-    unit_name: Optional[str] = None
-    unit_scale: Optional[np.float64] = None
+    __slots__ = ("_name", "_unit_name", "_unit_scale")
+
+    def __init__(
+        self,
+        name: str,
+        unit_name: Optional[str] = None,
+        unit_scale: Optional[np.float64] = None,
+    ):
+        self._name: str = name
+        self._unit_name: Optional[str] = unit_name
+        self._unit_scale: Optional[np.float64] = unit_scale
+
+    @property
+    def name(self) -> str:
+        """Name of the axis."""
+        return self._name
+
+    @property
+    def unit_name(self) -> Optional[str]:
+        """Optional string name for the physical quantity the units are measured in."""
+        return self._unit_name
+
+    @property
+    def unit_scale(self) -> Optional[np.float64]:
+        """Optional scale for the physical quantity the units."""
+        return self._unit_scale
 
 
 class CoordinateSpace(collections.abc.Sequence):