Filter out impossible SB <> SB and Flux <> Flux conversions in unit conversion plugin[BUG]

[cshanahan1]

Jdaviz component

Cubeviz

Description

If you load a MJy / sr cube, in the unit conversion plugin there are some units listed as available that you can't actually convert to (e.g MJy / sr to bol / sr, or erg / cm**2ssr). The unit conversion (which, from what I understand, only applies to the displayed data, not updating data in the data collection?) appears to happen, the display label is changed, but the flux disappears from the spectrum viewer because the conversion wasn't successful. This has implications for other plugins as well that use the flux / sb unit (like moment maps).

I'm not sure if this issue is limited to SB <> SB, i imagine it exists as well for flux <> flux.

How to Reproduce

1. Load cubeviz example notebook, which has data in MJy / sr
2. Use unit conversion plugin to convert to one of the available units that is invalid, like bol / sr

Expected behavior

These invalid options shouldn't be listed as possibilities.

Browser

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Jupyter

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Software versions

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