BF: many fixes for _scans.tsv files

[yarikoptic]

• assign metadata also to the ones under sessions
• n/a for empty values (as the bible says)
• correct time stamps per file
• having -ses_ suffix where applicable (BIDS manuscript also was fixed, validator works correctly)
• sanitize more fields in .json files (just in case)

[codecov-io]

Codecov Report

Merging #73 into master will increase coverage by 0.16%.
The diff coverage is 94.28%.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   78.26%   78.43%   +0.16%     
==========================================
  Files           6        6              
  Lines        1109     1127      +18     
==========================================
+ Hits          868      884      +16     
- Misses        241      243       +2

Impacted Files	Coverage Δ
tests/test_main.py	100% <100%> (ø)	⬆️
bin/heudiconv	74.06% <92.59%> (+0.14%)	⬆️

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[yarikoptic]

any feedback or a merge would be very welcome @satra? ;-)