Refactor metadata

[alessandrofelder]

Large PR with the goal to

• [x] write pynwb defaults to file if entry is optional, or "not_specified" if mandatory
• [x] avoid writing explanations of entry to file, and have them as tooltips in the GUI instead
• [x] fix issue with saving (introduced by ruamel ?)
• [x] double-check all signature changes are direct and correct consequence of yaml metadata changes
• [ ] write skeleton nwb file directly from a metadata.yaml file (put in later PR)

some issues/inelagancies of varying importance remain on the GUI side, but will be left as is by this PR:

• stimulus_details read as a list, not a dict?
• ~~comments only stored at lowest level of hierarchy (and therefore only displayed there)~~ we are fine with this, at least for now.
• optophys->location has actual comment code (not meant for tooltips) in it
• base yaml file read twice it's a short file anyway, and this is unlikely to change

[codecov-commenter]

Codecov Report

Merging #45 into master will increase coverage by 0.59%. The diff coverage is 53.24%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   51.62%   52.22%   +0.59%     
==========================================
  Files          11       11              
  Lines        1478     1507      +29     
  Branches      242      247       +5     
==========================================
+ Hits          763      787      +24     
- Misses        684      687       +3     
- Partials       31       33       +2     

Impacted Files	Coverage Δ
src/silverlabnwb/metadata_gui.py	0.00% <0.00%> (ø)
src/silverlabnwb/metadata.py	88.40% <89.18%> (+0.03%)	:arrow_up:
src/silverlabnwb/nwb_file.py	92.18% <100.00%> (+0.03%)	:arrow_up:
tests/test_epochs.py	85.45% <100.00%> (+0.13%)	:arrow_up:
src/silverlabnwb/signature.py	56.73% <0.00%> (-0.82%)	:arrow_down:

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