Workaround the LinearNDInterpolator thread-safety issue for Sentinel 1 SAR geolocation

[mraspaud]

• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff
• Fully documented

[coveralls]

Coverage increased (+0.02%) to 65.924% when pulling b712e53 on feature-multicore-sar-geography into e49c634 on master.

[codecov]

Codecov Report

Merging #302 into master will increase coverage by 0.02%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master     #302      +/-   ##
==========================================
+ Coverage    65.9%   65.92%   +0.02%     
==========================================
  Files         118      118              
  Lines       14332    14327       -5     
==========================================
  Hits         9445     9445              
+ Misses       4887     4882       -5

Impacted Files	Coverage Δ
satpy/readers/safe_sar_c.py	20% <22.22%> (+0.52%)	⬆️

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