First updates to IFU cube continuum notebook - need help to fix bugs

[camipacifici]

Started fixing this notebook to work with Jdaviz 4.0. I am finding a few bugs:

• spatial subsets defined from the API do not show in the viewer
• the wavelength axis gets loaded in meters, while it used to be loaded in angstrom (added a cell to do the conversion)
• model fitting crashes when defining a linear model for the full cube saying that '1 / pix2' and '' (dimensionless) are not convertible

I stopped there for now.

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[camipacifici]

Update, with current main, the cube model fitting fails with UnitConversionError: 'ct' and 'ct [/](http://localhost:8889/) pix2' are not convertible.

The wavelength axis now gets loaded fine in Angstrom.

[gibsongreen]

Update, with current main, the cube model fitting fails with UnitConversionError: 'ct' and 'ct [/](http://localhost:8889/) pix2' are not convertible.

The wavelength axis now gets loaded fine in Angstrom.

With the following draft PR, the specutils spec.with_flux_unit call should now have access to the pix2 unit equivalencies to make this conversion possible.
spacetelescope/jdaviz#3343

On a separate note, there is also a bug resolved from this draft PR that occurs when toggling 'Cube Fit' in Model Fitting that results in the following error:
TypeError: unsupported operand type(s) for *: 'CompositeUnit' and 'NoneType'