Fix HDF4 handling of scalar attributes

[djhoese]

I'm not sure if this is caused by a new version of numpy, changes to the dtype handling that I did a while back in the HDF4 file handler, or something else. The HDF4 utility file handler is returning all attributes as numpy arrays even if they are scalar attributes. This PR should fix this.

The other option I thought about doing is adding an if statement earlier in the attribute function that checks if the data isn't a numpy array then don't touch it. I'm not sure if this would be good or bad.

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage increased (+0.03%) to 86.783% when pulling 9d7cad0 on djhoese:bugfix-clavrx-3 into bf60e6a on pytroll:master.

[codecov]

Codecov Report

Merging #969 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #969      +/-   ##
==========================================
+ Coverage   86.76%   86.78%   +0.01%     
==========================================
  Files         179      179              
  Lines       27300    27305       +5     
==========================================
+ Hits        23688    23696       +8     
+ Misses       3612     3609       -3

Impacted Files	Coverage Δ
satpy/readers/hdf4_utils.py	96.49% <100%> (+3.76%)	⬆️
satpy/tests/reader_tests/test_hdf4_utils.py	92.53% <100%> (+0.34%)	⬆️
satpy/scene.py	90.53% <0%> (+0.17%)	⬆️

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