fix(schema): Minor issues

[effigies]

This removes a mistargeted issue that was inappropriately warning about EchoTime above and beyond the normal recommendation.

I also remove some required fields from the schema context to recognize that some fields may be absent without meaning the whole file is ill-formed.

Finally, I increase the specificity of some selectors for sidecar metadata to decrease the chance a validator will complain about missing metadata on a file that does not need it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.10%. Comparing base (b91d0e3) to head (c6bed8c).
Report is 145 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1893   +/-   ##
=======================================
  Coverage   88.10%   88.10%           
=======================================
  Files          16       16           
  Lines        1396     1396           
=======================================
  Hits         1230     1230           
  Misses        166      166           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.