Use rasterio to save geotiffs when available

[djhoese]

This refactors the geotiff writer in a couple new ways:

1. Replace floating_point with dtype which expects a numpy dtype and will figure everything else out after that.
2. Adds a check to see if rasterio is available and if so use the XRImage.save method for better performance.

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

[stickler-ci]

F401 'rasterio' imported but unused

[coveralls]

Coverage increased (+0.07%) to 63.967% when pulling 4e7a699 on feature-geotiff-refactor into 1fe7e8c on develop.

[codecov-io]

Codecov Report

Merging #252 into develop will decrease coverage by 2.58%.
The diff coverage is 71.01%.

@@             Coverage Diff             @@
##           develop     #252      +/-   ##
===========================================
- Coverage    63.89%   61.31%   -2.59%     
===========================================
  Files          112      111       -1     
  Lines        13858    13704     -154     
===========================================
- Hits          8855     8402     -453     
- Misses        5003     5302     +299

Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_geotiff.py	94.33% <ø> (ø)	⬆️
satpy/writers/geotiff.py	72.22% <71.01%> (+8.74%)	⬆️
satpy/composites/viirs.py	0% <0%> (-78.7%)	⬇️
satpy/readers/viirs_sdr.py	80.72% <0%> (-0.1%)	⬇️
satpy/tests/compositor_tests/__init__.py	98.46% <0%> (-0.03%)	⬇️
satpy/composites/crefl_utils.py	0% <0%> (ø)	⬆️
satpy/tests/compositor_tests/test_viirs.py

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[mraspaud]

LGTM