Added the NTIDIGITS18 dataset

[narduzzi]

No description provided.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 68.75000% with 20 lines in your changes missing coverage. Please review.

Project coverage is 77.32%. Comparing base (a4e8a45) to head (50a80d3).
Report is 85 commits behind head on develop.

Files with missing lines	Patch %	Lines
tonic/datasets/ntidigits18.py	68.25%	20 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #297      +/-   ##
===========================================
+ Coverage    77.07%   77.32%   +0.25%     
===========================================
  Files           54       56       +2     
  Lines         3067     3295     +228     
===========================================
+ Hits          2364     2548     +184     
- Misses         703      747      +44     

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[narduzzi]

Should the download from Dropbox be integrated in 'download_utils.py' instead of being local to this class?

[biphasic]

@narduzzi that looks very good! will merge right away