Issue 144 remove demo folder

[pvk-developer]

• atm get_demos returns a full list of the datasets available in our public bucket.
• atm download_demo <name of dataset> downloads a dataset (or a list of datasets).
• Also available in python with data.get_demos, returns a list, and data.download_demo returns the path to the saved dataset (or a list with paths if we requested a list of datasets).

Resolve #144

[codecov-io]

Codecov Report

Merging #148 into master will increase coverage by 2.02%.
The diff coverage is 73.8%.

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
+ Coverage   63.31%   65.34%   +2.02%     
==========================================
  Files          15       15              
  Lines        1723     1760      +37     
==========================================
+ Hits         1091     1150      +59     
+ Misses        632      610      -22

Impacted Files	Coverage Δ
atm/cli.py	17.92% <50%> (+17.92%)	⬆️
atm/data.py	67.74% <91.66%> (+8.58%)	⬆️

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