Add example creating GWCS from quantities and times.

[samaloney]

PR Description

Add an example showing how to create a GWCS and NDCube from quantities and times.

[Cadair]

This is interesting, I hadn't even thought of doing this.

That being said, I think I would prefer we document direct use of the Table coordinate classes like in the recent example of the AIA wavelength stack.

[samaloney]

So like this wcs = CompoundLowLevelWCS(energy_coord.wcs, time_coord.wcs)
the only thing here implicit order to get the right mapping between pixel and physical it a bit more explicit with the ExtraCoords?

[Cadair]

Suggested change

	wcs = CompoundLowLevelWCS(time_coord.wcs, energy_coord.wcs, )
	wcs = (time_coord & energy_coord).wcs

Would be the idiomatic way to do this iirc.

[samaloney]

Oh nice I might leave the more verbose way was a comment or comment or something

[samaloney]

So the cube at the end is the same but the object that created is a different, I've added both for the moment but only use the latter.

wcs = CompoundLowLevelWCS(time_coord.wcs, energy_coord.wcs)
wcs
<ndcube.wcs.wrappers.compound_wcs.CompoundLowLevelWCS object at 0x156ecce50>
CompoundLowLevelWCS Transformation
This transformation has 2 pixel and 2 world dimensions
Array shape (Numpy order): None
Pixel Dim  Axis Name  Data size  Bounds
        0  None            None  None
        1  None            None  None
World Dim  Axis Name  Physical Type  Units
        0  time       time           s
        1  energy     em.energy      keV
Correlation between pixel and world axes:
           Pixel Dim
World Dim    0    1
        0  yes   no
        1   no  yes

wcs2 = (time_coord & energy_coord).wcs
<WCS(output_frame=CompositeFrame, input_frame=PixelFrame, forward_transform=Model: CompoundModel
Inputs: ('x0', 'x1')
Outputs: ('y0', 'y1')
Model set size: 1
Expression: [0] & [1]
Components: 
    [0]: <Tabular1D(points=(<Quantity [0., 1., 2., 3., 4., 5., 6., 7., 8.] pix>,), lookup_table=[0. 1. 2. 3. 4. 5. 6. 7. 8.] s)>
    [1]: <Tabular1D(points=(<Quantity [0., 1., 2., 3., 4., 5., 6., 7., 8., 9.] pix>,), lookup_table=[0. 1. 2. 3. 4. 5. 6. 7. 8. 9.] keV)>
Parameters:)>

[Cadair]

I wouldn't recommend using CompoundLowLevelWCS unless you really have to. It creates a wrapper layer between two gWCS objects where it really should only be one gWCS object. Also there seems to be some serious performance overhead in CompoundLowLevelWCS, which I have never investigated. It's main use case is to join two different APE 14 WCSes together.

[samaloney]

Ok I've removed the CompoundLowLevelWCS approach.