Sb/prepifg investigation

[codecov-io]

Codecov Report

Merging #262 into develop will increase coverage by 1.51%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #262      +/-   ##
===========================================
+ Coverage    88.97%   90.48%   +1.51%     
===========================================
  Files           26       26              
  Lines         3364     3395      +31     
  Branches       574      575       +1     
===========================================
+ Hits          2993     3072      +79     
+ Misses         256      210      -46     
+ Partials       115      113       -2     

Impacted Files	Coverage Δ
pyrate/prepifg.py	99.09% <100.00%> (+46.67%)	:arrow_up:
pyrate/core/shared.py	93.60% <0.00%> (+0.29%)	:arrow_up:

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[mcgarth]

In this PR:

• the method of using GDAL system calls in prepifg (activated by largetifs: 1) is improved, but not totally fixed. There are still some minor numerical differences between this method and the direct python prepifg method. As a result, largetifs: 1 will be removed from user visibility in the release.
• the number of regression tests is reduced, in order that Travis tests pass within the 50 minute runtime limit.