Fix the cf_writer to accept single-valued time coordinate variable

[mraspaud]

This PR adds flexibility to the cf_writer. It fixes the time coordinate when it's a single value, and adds it as a dimension if possible.

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

[coveralls]

Coverage increased (+0.5%) to 74.621% when pulling 5f9d083 on mraspaud:fix-cf-writer into 19e3f83 on pytroll:master.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@12298a9). Click here to learn what that means.
The diff coverage is 96.55%.

@@            Coverage Diff            @@
##             master     #508   +/-   ##
=========================================
  Coverage          ?   76.51%           
=========================================
  Files             ?      136           
  Lines             ?    18904           
  Branches          ?        0           
=========================================
  Hits              ?    14464           
  Misses            ?     4440           
  Partials          ?        0

Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_cf.py	98.21% <100%> (ø)
satpy/writers/cf_writer.py	68.84% <91.66%> (ø)

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[mraspaud]

This seems to be an issue in xarray >= 0.10.4 pydata/xarray#2563