Make sure a discrete data type is preserved through resampling

[mraspaud]

This PR lets satpy resample discrete dataset without converting to float. Additionally, if a _FillValue attribute is available, that value is passed to pyresample for filling missing data pixels (typically outside of swath).

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

[djhoese]

Just a few "gotchas" that I think could be issues.

[coveralls]

Coverage increased (+0.4%) to 74.474% when pulling bc6ec5e on mraspaud:fix-dtype-preservation into 19e3f83 on pytroll:master.

[codecov]

Codecov Report

Merging #510 into master will increase coverage by 0.34%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #510      +/-   ##
==========================================
+ Coverage   74.13%   74.47%   +0.34%     
==========================================
  Files         137      136       -1     
  Lines       18257    18291      +34     
==========================================
+ Hits        13534    13622      +88     
+ Misses       4723     4669      -54

Impacted Files	Coverage Δ
satpy/tests/test_resample.py	98.61% <100%> (+0.11%)	⬆️
satpy/resample.py	81.6% <100%> (+5.09%)	⬆️
satpy/readers/hrit_base.py	79.5% <0%> (-1.92%)	⬇️
satpy/readers/hrit_electrol.py	34.42% <0%> (ø)	⬆️
satpy/readers/hrit_msg.py	44.63% <0%> (ø)	⬆️
satpy/readers/hrit_goes.py	51.65% <0%> (ø)	⬆️
satpy/readers/hrit_jma.py	97.67% <0%> (ø)	⬆️
satpy/readers/nc_nwcsaf_msg.py
satpy/tests/reader_tests/test_hrit_base.py	93.87% <0%> (+0.03%)	⬆️

---

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 19e3f83...bc6ec5e. Read the comment docs.