Support 2D plotting in NDCube using APE14

[yashrsharma44]

This PR addresses the rewriting of plotting.py for 2D NDCube.

Note this PR only modifies the tests for 1D plotting instead of making code changes, as the plotting of 1D cube works as it was working earlier.

This PR requires astrofrog's PR #8885 and plotting support in SlicedLowLevelWCS PR #8980 to be merged before this code is merged.
The above issue was sorted.

Edit:
The plotting of NDCube can be operated in 3 ways. The checklist denotes the feature which is working.

• Simple plotting - NDCube.plot().
• Plotting with plot_axis_indices argument - This feature is failing due to the bug in NDCube master. More info
• Plotting with axes_coordinates argument.

[pep8speaks]

Hello @yashrsharma44! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2019-08-20 14:52:06 UTC

[ghost]

Thanks for the pull request @yashrsharma44!

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