Make spectral-cube optional dependency

[pllim]

glue-astronomy pulls in spectral-cube no matter what but Jdaviz does not need it anymore. Also see:

• Remove dependency on spectral-cube spacetelescope/jdaviz#1006

We cannot make glue-astronomy an optional dependency but we also do not want spectral-cube to be installed unnecessarily.

[dhomeier]

@astrofrog do you have an idea how to conditionally enable a @data_translator? Can't think of an elegant way to do this.
At the same time #54 is a request to extend spectral-cube support.

[pllim]

This is still a pain for us. Occasionally we get help call about some dependency from this package that broke Jdaviz installation, some CASA IO stuff that we never use.

[dhomeier]

FWIW the "brute force" way of putting the entire translator into a

try:
    from spectral_cube import SpectralCube

just works; would only need to update the tests to probe for spectral_cube availability (or put it into the test extras_require require instead).

And maybe putting the whole import into a wrapper file somehow like in https://github.com/glue-viz/glue/blob/f8c1e9646e7252aa13f5f58c4838cb9b670875d3/glue/core/data_factories/dendrogram.py#L6-L7
would be a more elegant solution?

[pllim]

This is too hacky?

Change this:

glue-astronomy/glue_astronomy/translators/spectral_cube.py

Line 9 in bae4ecf

from spectral_cube import SpectralCube, BooleanArrayMask

To this:

try:
    from spectral_cube import SpectralCube, BooleanArrayMask
except ImportError:
    class SpectralCube:
        pass
    BooleanArrayMask = None

[dhomeier]

Definitely much shorter, looks OK to me as well. 😃
Was just wondering if some extra warning is due on the non-available translator, but since one could not even do a
data.get_object(SpectralCube, ...) I guess that would sort itself out.

[pllim]

Okay, I would like to apologize for blaming that on spectral-cube, it is actually glue-core. I'll open a separate issue about that.