Added cell description to tqdm

[GiovanniCiampi]

Added possibility to set description of cells that will be injected to the tqdm.
To add description to a specific cell the user must add a string with this formatting in the desired cell:
# papermill_description=description_of_cell
When the execution will reach that cell, the description of the tqdm will be changed from Executing x: to Executing description_of_cell.
If no cell contains the tag papermill_description=, the execution will be as before the implementation of this functionality.
If a cell containing the description tag is followed by cells that don't have the tag, the tqdm description is set to the last valid description.

I decided to implement this functionality because I think it is pretty useful with long notebooks that have very slow cells (i.e. data preparation and model training). An alternative can be to use logging, but it is way more verbose.
With this implementation you can simply add the 'Training' description to a cell and you know you are in the training process, or you can add 'Data_Preparation' and you know you are doing data preparation, moreover you see only the description of the current cell in the tqdm.
If a user doesn't like this functionality, he can simply ignore it.

I think it would be a good idea to add information about this feature in the documentation, but I would like to know first if this this is correctly implemented and perceived as a good addition from the authors/contributors.

[codecov]

Codecov Report

Merging #565 (c79c6a5) into main (a318730) will increase coverage by 0.54%.
The diff coverage is 70.00%.

❗ Current head c79c6a5 differs from pull request most recent head b515364. Consider uploading reports for the commit b515364 to get more accurate results

@@            Coverage Diff             @@
##             main     #565      +/-   ##
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+ Coverage   91.65%   92.19%   +0.54%     
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  Files          17       16       -1     
  Lines        1558     1423     -135     
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- Hits         1428     1312     -116     
+ Misses        130      111      -19     

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[MSeal]

Thanks for the addition, well get this into the next release

[MSeal]

Overall the approach seems fine to me for an opt-in pattern. There's not a great programmatic pattern for fetching first comment in a cell outside of loading a full AST parser that's comment aware. This is simpler and less dependency requiring to achieve the same thing.