Fix units in WCS API world_axis_object_components

[Cadair]

fixes #495

We were returning Quantity objects for CelestialFrame when we shouldn't have been, but we also need to make sure we cast the values to the units of the frame.

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@@            Coverage Diff             @@
##           master     #512      +/-   ##
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- Coverage   87.28%   87.25%   -0.03%     
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  Files          22       21       -1     
  Lines        3821     3813       -8     
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  Misses        486      486              

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[nden]

@Cadair I am trying to understand this. Can you verify that the example in the issue works now? When I run it I don't see a change with this PR. Running high_level_objects_to_values with that example does not return scalars or numpy arrays.

[Cadair]

hey @nden, here's an example:

Setup

from astropy.modeling import models
from gwcs import WCS
import gwcs.coordinate_frames as cf
import astropy.units as u
import astropy.coordinates as coord
from astropy.wcs.wcsapi.wrappers import SlicedLowLevelWCS
from astropy.wcs.wcsapi.high_level_api import high_level_objects_to_values
import numpy as np


identity = (models.Multiply(1 * u.arcsec / u.pixel) &
            models.Multiply(1 * u.arcsec / u.pixel) &
            models.Multiply(1 * u.nm / u.pixel))
sky_frame = cf.CelestialFrame(axes_order=(0, 1), name='icrs',
                              reference_frame=coord.ICRS(),
                              axes_names=("longitude", "latitude"),
                              unit=(u.arcsec, u.arcsec))
wave_frame = cf.SpectralFrame(axes_order=(2, ), unit=u.nm, axes_names=("wavelength",))

frame = cf.CompositeFrame([sky_frame, wave_frame])

detector_frame = cf.CoordinateFrame(name="detector", naxes=3,
                                    axes_order=(0, 1, 2),
                                    axes_type=("pixel", "pixel", "pixel"),
                                    axes_names=("x", "y", "z"), unit=(u.pix, u.pix, u.pix))

wcs = WCS(forward_transform=identity, output_frame=frame, input_frame=detector_frame)

values = high_level_objects_to_values(*wcs.pixel_to_world(1, 2, 3), low_level_wcs=wcs)

before:

[<Longitude 0.00027778 deg>, <Latitude 0.00055556 deg>, np.float64(3.0)]

after:

[np.float64(1.0), np.float64(2.0), np.float64(3.0)]

The issue being that before we had Quantity instances and they were not in frame units, now we have numbers in frame units.

[Cadair]

@nden are we going to be able to get this out in a release before the final release of astropy 7?

[nden]

I can get a release out but I was assumign after astropy 7.0. Or do you need this before?

[Cadair]

Well without this patch and with astropy 7 some things will break for me, so would be nice to get this out first?