changed all instances of "missing_axis" to "missing_axes"

[gnshafreak]

Description

Fixes #130

[DanRyanIrish]

ping @Cadair. Your dream come true ;)

[Cadair]

Thanks @gnshafreak I will take a look later 😄

[DanRyanIrish]

Hi @gnshafreak. This looks great. So that we don't break the API, you also add a missing_axis property to NDCubeBase? I should raise a DeprecationWarning saying that missing_axes has replaced missing_axis and ```missing_axis will no longer be supported in 2.0? So it would be something like this:

@property
def missing_axis(self):
    # Issue deprecation warning here with message "missing_axis has been deprecated by missing_axes and will no longer be supported after ndcube 2.0".
    return self.missing_axes

[ghost]

Thanks for the pull request @gnshafreak!

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[DanRyanIrish]

Hi @gnshafreak. This deprecation warning looks good! I think this may be ready to merge. Depending on the reason for the circleci test failure.

@Cadair @nabobalis pip seems to be failing to install something for the circleci tests. Is this a known issue with sunpy as well? https://circleci.com/gh/sunpy/ndcube/129?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

[nabobalis]

Yes, that is an upstream issue that should be fixed in the next pip release.

[DanRyanIrish]

Thanks @nabobalis! In that case, I think this PR is ready to merge! Any objects @Cadair?

[DanRyanIrish]

Delete this line.

[DanRyanIrish]

Add a file in the changelog/ directory called 157.feature.rst
Add the changelog entry in there.