Fix geotiff writer being unimportable if gdal isn't installed

[djhoese]

The geotiff writer is deprecating its use of the 'gdal' python library. It is recommended (and has been for a long time) that trollimage is used with rasterio. The rasterio library only uses the C gdal library and not the python library. On some environments (conda) it is possible to have rasterio and not python gdal. The geotiff writer currently imports gdal regardless of rasterio existing so in this case you would be unable to use the geotiff writer at all even though you had all the necessary dependencies. This PR fixes that and raises an error if neither rasterio or gdal is importable.

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

[codecov]

Codecov Report

Merging #616 into master will decrease coverage by 0.02%.
The diff coverage is 36.36%.

@@            Coverage Diff            @@
##           master    #616      +/-   ##
=========================================
- Coverage   78.33%   78.3%   -0.03%     
=========================================
  Files         136     136              
  Lines       19714   19722       +8     
=========================================
+ Hits        15442   15443       +1     
- Misses       4272    4279       +7

Impacted Files	Coverage Δ
satpy/writers/geotiff.py	42.01% <36.36%> (-2.13%)	⬇️

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[coveralls]

Coverage decreased (-0.03%) to 78.303% when pulling baf5f42 on djhoese:bugfix-geotiff-writer-import into f55d08a on pytroll:master.