Refactor the combine_metadata function and allow numpy arrays to be combined

[mraspaud]

The valid_range array was not being combined properly because each dataarray had it's own copy of it. This refactor changes the behaviour of combine_metadata to just check the identities of lazy arrays, and use np.allclose otherwise.

• Tests added
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage increased (+0.02%) to 90.037% when pulling 1823109 on mraspaud:fix-np-scalar-combine into 4924cba on pytroll:master.

[codecov]

Codecov Report

Merging #1309 into master will increase coverage by 0.01%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master    #1309      +/-   ##
==========================================
+ Coverage   90.01%   90.03%   +0.01%     
==========================================
  Files         220      220              
  Lines       32254    32309      +55     
==========================================
+ Hits        29035    29090      +55     
  Misses       3219     3219              

Impacted Files	Coverage Δ
satpy/dataset.py	92.93% <94.50%> (+0.32%)	⬆️
satpy/tests/test_dataset.py	100.00% <100.00%> (ø)

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[djhoese]

I think this looks good, but honestly it is hard to review with this new coding style. I know the code is overall more organized but I have to jump around between so many functions...this looks fine. I think originally Gerrit and I were trying to avoid every comparing numpy arrays. Not sure if that was how it was actually implemented before, but I'm fine with what you've done here.

[mraspaud]

I go from the assumption that heavy arrays will be lazy, so pure numpy arrays are OK to compare. As you can see in the tests, this was to compare eg valid_range 2-element arrays. Another solution would be to set a hardcoded limit on the length of arrays that we are OK with comparing.

[djhoese]

Another solution would be to set a hardcoded limit on the length of arrays that we are OK with comparing.

Might be a good idea in the future. I wish xarray would return valid_range as a tuple/list instead of an array. It will always be two elements.

[mraspaud]

Regarding jumping between functions: I tried to always put a function's definition after it was used, so that we can go from high level to low level by reading from top to bottom. But yeah, reviewing isn't probably as linear anymore, but hopefully the intent and implementation of the functions is clearer now.