Use builtin Astropy model unit and bounding box support

Define the `input_units`, `return_units`, `input_units_equivalencies`,
and `bounding_box` properties for all models. Use Astropy models' built-in
unit conversion support.

All models now require inputs with valid units (wavelength, wavenumber, or
frequency). Dimensionless inputs are no longer automatically converted to
wavenumber.

Fixes #237.

CHANGES.rst

@@ -1,7 +1,13 @@
 1.6.dev (unreleased)
 ================
 
-- none yet
+- Define the ``input_units``, ``return_units``, ``input_units_equivalencies``,
+  and ``bounding_box`` properties for all models. Use Astropy models' built-in
+  unit conversion support.
+
+- All models now require inputs with valid units (wavelength, wavenumber, or
+  frequency). Dimensionless inputs are no longer automatically converted to
+  wavenumber.
 
 1.5 (2024-08-16)
 ================

docs/dust_extinction/dev_model.rst

@@ -16,7 +16,7 @@ All
 All dust extinction models have at least the following:
 
 * A member variable `x_range` that that define the valid range of wavelengths. These are defined in inverse microns as is common for extinction curve research.
-* A member function `evaluate` that computes the extinction at a given `x` and any model parameter values.  The `x` values are checked to be within the valid `x_range`. The `x` values should have astropy.units.  If they do not, then they are assumed to be in inverse microns and a warning is issued stating such.
+* A member function `evaluate` that computes the extinction at a given `x` and any model parameter values.  The `x` values are checked to be within the valid `x_range`. The `x` values should have astropy.units.
 
 All of these classes used in ``dust_extinction`` are based on the 
 `Model <https://docs.astropy.org/en/stable/modeling/>`_ astropy.modeling class.