Remove pyximports from gradient search

[mraspaud]

pyximports make the import of gradient resampler fail if cython isn't installed

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

[coveralls]

Coverage decreased (-0.002%) to 91.253% when pulling 0857d67 on mraspaud:fix-gradient into 4afcc5f on pytroll:master.

[codecov]

Codecov Report

Merging #245 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
- Coverage   91.27%   91.27%   -0.01%     
==========================================
  Files          42       42              
  Lines        8245     8243       -2     
==========================================
- Hits         7526     7524       -2     
  Misses        719      719

Impacted Files	Coverage Δ
pyresample/resampler.py	34.78% <ø> (ø)	⬆️
pyresample/test/test_gradient.py	93.12% <ø> (ø)	⬆️
pyresample/gradient/__init__.py	94.49% <ø> (-0.1%)	⬇️

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