Updating FITS-cubes tutorial in quarantine

[kakirastern]

To follow up on Issue #289.

Two goals in mind:

1. Update code blocks to replace outdated code snippets
2. Reduce cube file size to manageable level

[kakirastern]

Following up on Goal No. 1:
I have checked that all the code blocks in the file work after the first commit b75aaf5. But reckon the "Find and Download a Herschel Image" part takes a bit long to download the desired images, and it may need a bit of updating as the results obtained with ESASky.query_region_maps() is now up to 28 from the previous 25. It took me more than 30 minutes to complete the process of downloading the images on my laptop. But personally I prefer using ESASky.query_object_maps() over ESASky.query_region_maps() to search for the SMC images.

[kakirastern]

Revised file with significant changes currently available on colab at view link.

[kakirastern]

I really liked my experience testing the workflow on Google Colab... the downloading took only several minutes max as opposed to the >30min on my own laptop, and I could easily import any missing packages with pip. The flow was really smooth...

[kakirastern]

Have reduced the original cube file 'TAN_C14.fits' from 280.8 MB to 40.5 MB in size.

[kakirastern]

Goal 2 has also been accomplished. The reduced cube can now be viewed at Google Drive link. The download time is now less than 5 minutes, approximately.

Please let me know if any further modification is required.

[kakirastern]

The checked FITS-cubes file has been tested with the following packages as of Tuesday, Feb. 12th, 2019:

astropy: 3.1.1
numpy: 1.15.4
matplotlib: 3.0.2
ipython: 7.2.0
aplpy: 1.1.1
spectral-cube: 0.4.3
reproject: 0.4
astroquery: 0.3.9

[kakirastern]

Once the reduced-sized datacube has been approved, I will modify the FITS-cubes tutorial accordingly.

[kakirastern]

The PR is ready for a review.