Fixup units check (GalacticDynamics#34)

* fix units check in pytree
* get rid of units default

Signed-off-by: nstarman <nstarman@users.noreply.github.com>

src/galdynamix/potential/_potential/core.py

@@ -15,9 +15,7 @@
 
 class AbstractPotential(AbstractPotentialBase):
     _: KW_ONLY
-    units: UnitSystem = eqx.field(
-        default=None, converter=converter_to_usys, static=True
-    )
+    units: UnitSystem = eqx.field(converter=converter_to_usys, static=True)
     _G: float = eqx.field(init=False, static=True, repr=False, converter=float)
 
     def __post_init__(self) -> None:

src/galdynamix/potential/_potential/param/field.py

@@ -79,6 +79,12 @@ def __get__(  # TODO: use `Self` when beartype is happy
 
     def _check_unit(self, potential: AbstractPotential, unit: Unit) -> None:
         """Check that the given unit is compatible with the parameter's."""
+        # When the potential is being constructed, the units may not have been
+        # set yet, so we don't check the unit.
+        if not hasattr(potential, "units"):
+            return
+
+        # Check the unit is compatible
         if not unit.is_equivalent(
             potential.units[self.dimensions],
             equivalencies=self.equivalencies,
@@ -98,7 +104,8 @@ def __set__(
         if isinstance(value, AbstractParameter):
             # TODO: this doesn't handle the correct output unit, a. la.
             # potential.units[self.dimensions]
-            self._check_unit(potential, value.unit)  # Check the unit is compatible
+            # Check the unit is compatible
+            self._check_unit(potential, value.unit)
         elif callable(value):
             # TODO: this only gets the existing unit, it doesn't handle the
             # correct output unit, a. la. potential.units[self.dimensions]