Fix image overlays for single band data (requires trollimage 1.6+)

[djhoese]

Requires pytroll/trollimage#28 to be merged and trollimage 1.6 to be released.

Additionally the documentation currently fails to build because pydecorate hasn't had a passing sphinx build on readthedocs in over a year. I think once it is built with a semi-recent version of sphinx it should include the necessary intersphinx inventory files on readthedocs.

• Closes Adding coastlines to single channel not working #449
• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[codecov]

Codecov Report

Merging #551 into master will increase coverage by 0.44%.
The diff coverage is 89.09%.

@@            Coverage Diff             @@
##           master     #551      +/-   ##
==========================================
+ Coverage   76.81%   77.26%   +0.44%     
==========================================
  Files         136      136              
  Lines       19029    19124      +95     
==========================================
+ Hits        14618    14776     +158     
+ Misses       4411     4348      -63

Impacted Files	Coverage Δ
satpy/tests/test_writers.py	97.81% <100%> (+0.26%)	⬆️
satpy/writers/__init__.py	86.18% <70%> (+9.46%)	⬆️
satpy/tests/test_readers.py	98.16% <0%> (-0.44%)	⬇️
satpy/readers/__init__.py	94.38% <0%> (+0.03%)	⬆️
satpy/readers/hrit_base.py	80.74% <0%> (+1.24%)	⬆️
satpy/readers/yaml_reader.py	92.57% <0%> (+3.57%)	⬆️
satpy/readers/seviri_l1b_hrit.py	51.41% <0%> (+6.77%)	⬆️

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[coveralls]

Coverage decreased (-0.05%) to 76.642% when pulling 97ef3da on djhoese:feature-single-band-coastlines into 129cba2 on pytroll:master.

[coveralls]

Coverage increased (+0.4%) to 77.243% when pulling 78f9125 on djhoese:feature-single-band-coastlines into 2f931e3 on pytroll:master.

[mraspaud]

Really nice to have this feature implemented, thanks!

[djhoese]

Pydecorate's documentation has now been rebuilt and the intersphinx links now pass however the reader table looks like it has some indentation issues. I'll fix that in the related reader rename PR.

[djhoese]

Tests are currently failing due to a bug in trollimage 1.6.0 that will be fixed in 1.6.1 via pytroll/trollimage#34