Simplify header syntax

[jesper-friis]

Description

Allowed to omit initial ?-sign in table header with no label. For example, allow the user to write "length[unit=m]" instead of "length[?unit=m]" in a table header.

Also updated the documentation accordingly.

Type of change

• Bug fix and code cleanup
• New feature
• Documentation update
• Testing

Checklist for the reviewer

This checklist should be used as a help for the reviewer.

• Is the change limited to one issue?
• Does this PR close the issue?
• Is the code easy to read and understand?
• Do all new feature have an accompanying new test?
• Has the documentation been updated as necessary?
• Is the code properly tested?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.54%. Comparing base (00df659) to head (9755901).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #562   +/-   ##
=======================================
  Coverage   85.54%   85.54%           
=======================================
  Files          30       30           
  Lines        5118     5121    +3     
=======================================
+ Hits         4378     4381    +3     
  Misses        740      740           

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[francescalb]

Looks OK, but please add an example with using both label and key, so that it becomes clearer that this is also allowed.