Add fits option to meta.to_dict, silences warnings

[maxnoe]

Before the log was full of warnings related to implicitly relying on conversion of too long header keywords using HIERARCH. This adds an fits option to the to_dict method to explicitly add HIERARCH to silence these warnings.

This did not remove all warnings however, since:

  /home/maxnoe/Uni/CTA/ctapipe/ctapipe/io/tests/test_metadata.py:32: DeprecationWarning: Passing unrecoginized arguments to super(Process).__init__(_type='Simulation', _id=423442).
  object.__init__() takes exactly one argument (the instance to initialize)
  This is deprecated in traitlets 4.2.This error will be raised in a future release of traitlets.
    process=meta.Process(_type="Simulation", subtype="Prod3b", _id=423442,),

ctapipe/io/tests/test_metadata.py::test_construct_and_write_metadata
  /home/maxnoe/.local/anaconda/envs/cta-dev/lib/python3.7/site-packages/astropy/io/fits/convenience.py:589: AstropyUserWarning: Attribute `HIERARCH CTA PRODUCT CREATION TIME` of type <class 'astropy.time.core.Time'> cannot be added to FITS Header - skipping
    f"added to FITS Header - skipping", AstropyUserWarning)

ctapipe/io/tests/test_metadata.py::test_construct_and_write_metadata
  /home/maxnoe/.local/anaconda/envs/cta-dev/lib/python3.7/site-packages/astropy/io/fits/convenience.py:589: AstropyUserWarning: Attribute `HIERARCH CTA ACTIVITY START TIME` of type <class 'astropy.time.core.Time'> cannot be added to FITS Header - skipping
    f"added to FITS Header - skipping", AstropyUserWarning)

These warnings were probably lost in the large amounts of HIERARCH related warnings

[codecov]

Codecov Report

Merging #1433 into master will increase coverage by 0.10%.
The diff coverage is 93.73%.

@@            Coverage Diff             @@
##           master    #1433      +/-   ##
==========================================
+ Coverage   90.91%   91.01%   +0.10%     
==========================================
  Files         184      184              
  Lines       12677    12957     +280     
==========================================
+ Hits        11525    11793     +268     
- Misses       1152     1164      +12     

Impacted Files	Coverage Δ
ctapipe/image/extractor.py	86.16% <0.00%> (-0.45%)	⬇️
ctapipe/image/tests/test_reducer.py	100.00% <ø> (ø)
ctapipe/tools/bokeh/file_viewer.py	75.63% <50.00%> (ø)
ctapipe/tools/display_integrator.py	96.40% <50.00%> (-0.03%)	⬇️
ctapipe/io/eventseeker.py	75.00% <85.71%> (-6.82%)	⬇️
ctapipe/instrument/subarray.py	94.50% <87.27%> (-3.14%)	⬇️
ctapipe/tools/tests/test_tools.py	97.46% <87.87%> (-2.54%)	⬇️
ctapipe/calib/camera/calibrator.py	100.00% <100.00%> (ø)
ctapipe/containers.py	100.00% <100.00%> (ø)
ctapipe/core/component.py	96.72% <100.00%> (+0.23%)	⬆️
... and 28 more

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[maxnoe]

With this, we are down to a manageble amount of warnings in the tests so it should be easy to spot new ones.