Check whether time dimension exists for timeseries

[zxdawn]

If time dimension exists, we can blend them directly.

• Tests added
• Tests passed
• Passes flake8 satpy
• Fully documented
• Add your name to AUTHORS.md if not there already

[coveralls]

Coverage increased (+0.4%) to 89.956% when pulling 0c04bf7 on zxdawn:multiscene into 65cd83f on pytroll:master.

[codecov]

Codecov Report

Merging #1173 into master will increase coverage by 0.34%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1173      +/-   ##
==========================================
+ Coverage   89.60%   89.95%   +0.34%     
==========================================
  Files         200      208       +8     
  Lines       29484    30596    +1112     
==========================================
+ Hits        26420    27523    +1103     
- Misses       3064     3073       +9     

Impacted Files	Coverage Δ
satpy/multiscene.py	89.63% <100.00%> (+0.30%)	⬆️
satpy/tests/test_multiscene.py	99.39% <100.00%> (+0.13%)	⬆️
satpy/readers/tropomi_l2.py	87.06% <0.00%> (-2.94%)	⬇️
satpy/readers/avhrr_l1b_gaclac.py	93.33% <0.00%> (-1.34%)	⬇️
satpy/readers/viirs_compact.py	64.12% <0.00%> (-1.27%)	⬇️
satpy/readers/hrit_jma.py	97.94% <0.00%> (-1.22%)	⬇️
satpy/tests/test_composites.py	99.88% <0.00%> (-0.12%)	⬇️
satpy/writers/cf_writer.py	93.77% <0.00%> (-0.03%)	⬇️
satpy/writers/geotiff.py	90.74% <0.00%> (ø)
satpy/tests/test_dataset.py	100.00% <0.00%> (ø)
... and 43 more

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[mraspaud]

LGTM. Do we haves tests for multiscene? In which case a small test for this would be nice to have

[zxdawn]

@mraspaud I've added the test to the part of test_timeseries in test_multiscene.py.

[mraspaud]

LGTM

[zxdawn]

@djhoese Is this PR fine enough?

[mraspaud]

@zxdawn Sorry I forgot to merge this.