Remove pymc3/examples

[MarcoGorelli]

xref #4348 (comment)

This PR does a few things:

• the pymc3/examples/data files which were used in tests have been moved to pymc3/tests/data, so that those tests (pymc3/tests/test_examples.py) can continue running as before
• any other file which might have been accessed by pm.get_data is now in pymc-examples - if someone tries to open it using pm.get_data, it'll be downloaded from pymc-examples. This keeps get_data backwards-compatible whilst lightening the PyPI package
• all other files have been removed from pymc3/examples, as they're already in the pymc-examples repo

[codecov]

Codecov Report

Merging #4352 (c2e31aa) into master (4e5edd5) will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #4352      +/-   ##
==========================================
- Coverage   87.95%   87.93%   -0.02%     
==========================================
  Files          88       88              
  Lines       14499    14506       +7     
==========================================
+ Hits        12752    12756       +4     
- Misses       1747     1750       +3     

Impacted Files	Coverage Δ
pymc3/data.py	80.26% <66.66%> (-0.75%)	⬇️

[fonnesbeck]

LGTM