Add integration test

[HealthyPear]

This PR adds an integration test that runs the EventDisplay example.

It is part of issue #24.

Travis now runs the example from this test which:

• check that the output file exists and is not-empty,
• uses the ogadf-schema utilities to check that the HDUs are in the correct format (issue Use ogadf-schema to check if output is compliant to gadf #43)

Note
This test operates on the current test data which corresponds to the on-axis case.
Once we'll set up the off-axis DL2 test data to download, we'll need a new script (even by copying the current one).
That script will require its own integration test, so we should have in the end and integration test for point-like IRFs and one for full-enclosure ones.

[codecov]

Codecov Report

Merging #96 into master will increase coverage by 0.32%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
+ Coverage   87.17%   87.50%   +0.32%     
==========================================
  Files          35       36       +1     
  Lines        1138     1168      +30     
==========================================
+ Hits          992     1022      +30     
  Misses        146      146              

Impacted Files	Coverage Δ
pyirf/io/tests/test_gadf.py	98.88% <ø> (-0.05%)	⬇️
pyirf/io/gadf.py	100.00% <100.00%> (ø)
pyirf/tests/test_eventdisplay_comparison.py	100.00% <100.00%> (ø)

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