Fix resampler imports

[pnuu]

This PR moves the pyresample resampler imports inside the methods they are used. In this way users won't be forced to upgrade pyresample if they don't need newer resamplers and features.

NOTE: for bilinear interpolation the pyresample upgrade is still necessary, and so is if bucket resampler is needed. The default nearest resampler will work as expected also with older (< 1.13.0) pyresample versions without forcing an update.

• Closes satpy 0.17 does not work with pyresample 1.11 #918
• Tests updated
• Tests passed
• Passes flake8 satpy

[djhoese]

Nice!

[coveralls]

Coverage decreased (-0.004%) to 85.348% when pulling b4acd1b on pnuu:fix-resampler-imports into 95b0aea on pytroll:master.

[codecov]

Codecov Report

Merging #927 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #927   +/-   ##
=======================================
  Coverage   85.35%   85.35%           
=======================================
  Files         174      174           
  Lines       26037    26037           
=======================================
  Hits        22223    22223           
  Misses       3814     3814

Impacted Files	Coverage Δ
satpy/tests/test_resample.py	98.37% <100%> (ø)	⬆️
satpy/resample.py	92.97% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 95b0aea...b4acd1b. Read the comment docs.

[mraspaud]

LGTM