Establish coverage diagnostics

[khaeru]

This PR adds:

• A simple function item.historical.diagnostic.coverage() that displays some description of the areas/countries and periods covered by a data set.
• Tests and a command-line command for running this function.
• Configuration to:
  • Run these diagnostics on continuous integration (CI) builds. This means the diagnostics are re-run automatically every time the code is changed.
  • Upload the diagnostic outputs to cloud storage. E.g. for Travis build #313, the diagnostic output is at: https://storage.googleapis.com/historical-data-ci.transportenergy.org/313.1/index.html
  • Post a message in our Slack channel with a link to these outputs.
• [x] Update documentation.
• [x] Also zip and upload the cached input data (requested by @soniayeh).

These pieces are the basis for expanded coverage diagnostics, e.g. plots and graphics, and application to mid-stream and final output data.

Closes #14.

[codecov[bot]]

Codecov Report

Merging #23 into master will decrease coverage by 0.06%. The diff coverage is 50.90%.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
- Coverage   50.03%   49.96%   -0.07%     
==========================================
  Files          32       33       +1     
  Lines        1425     1507      +82     
==========================================
+ Hits          713      753      +40     
- Misses        712      754      +42     

Impacted Files	Coverage Δ
item/remote/sdmx.py	100.00% <ø> (ø)
item/tests/test_cli.py	0.00% <ø> (ø)
item/tests/test_historical.py	0.00% <0.00%> (ø)
item/historical/diagnostic.py	53.84% <53.84%> (ø)
item/historical/cli.py	65.51% <60.00%> (+3.61%)	:arrow_up:
item/historical/__init__.py	37.00% <100.00%> (+1.28%)	:arrow_up:
item/remote/cli.py	100.00% <0.00%> (ø)
item/tests/conftest.py	0.00% <0.00%> (ø)
item/tests/test_model.py	0.00% <0.00%> (ø)
... and 8 more

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