Datafind protection for running scripts on laptops

[alurban]

This PR adds a JSON exception to the logic of gwdetchar.io.datafind.get_data, which is needed to be able to run gwdetchar scripts on a laptop.

cc @duncanmmacleod

[coveralls]

Coverage decreased (-0.5%) to 94.987% when pulling ee7f058 on alurban:fix-datafind into fd8de27 on gwdetchar:master.

[coveralls]

Coverage decreased (-0.5%) to 94.987% when pulling ee7f058 on alurban:fix-datafind into fd8de27 on gwdetchar:master.

[coveralls]

Coverage increased (+0.01%) to 95.51% when pulling 11352d3 on alurban:fix-datafind into fd8de27 on gwdetchar:master.

[codecov-io]

Codecov Report

Merging #300 into master will increase coverage by 0.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master     #300      +/-   ##
==========================================
+ Coverage   95.53%   95.54%   +0.01%     
==========================================
  Files          21       21              
  Lines        1522     1526       +4     
==========================================
+ Hits         1454     1458       +4     
  Misses         68       68

Flag	Coverage Δ
#Linux	95.54% <80%> (+0.01%)	⬆️
#python27	95.22% <80%> (+0.01%)	⬆️
#python35	94.69% <40%> (-0.12%)	⬇️
#python36	94.82% <40%> (-0.12%)	⬇️
#python37	94.76% <40%> (-0.12%)	⬇️

Impacted Files	Coverage Δ
gwdetchar/io/datafind.py	96.43% <80%> (+0.27%)	⬆️

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