Read the meta_data dictionary from pygac

[ninahakansson]

Read meta_data attribute from pygac

• Tests passed
• Passes flake8 satpy
• Fully documented
• Wait for pygac > 1.2.1

[coveralls]

Coverage increased (+0.0005%) to 86.865% when pulling 949e368 on ninahakansson:read_pygac_meta_data into 177d150 on pytroll:master.

[codecov]

Codecov Report

Merging #980 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #980      +/-   ##
==========================================
- Coverage   86.86%   86.86%   -0.01%     
==========================================
  Files         181      181              
  Lines       27461    27462       +1     
==========================================
  Hits        23854    23854              
- Misses       3607     3608       +1

Impacted Files	Coverage Δ
satpy/readers/avhrr_l1b_gaclac.py	94.85% <100%> (ø)	⬆️
satpy/tests/reader_tests/test_avhrr_l1b_gaclac.py	97.98% <100%> (+0.01%)	⬆️
satpy/scene.py	90.35% <0%> (-0.18%)	⬇️

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[mraspaud]

LGTM