Move DataContent to multio - only place where it is used

[pgeier]

Description

The UserDataContent and DataContent have been removed. Their only usage is in multio.
As the implementation is very minimal, it has been moved to multio to reduce dependency overhead.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.76%. Comparing base (b618639) to head (b221149).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #131      +/-   ##
===========================================
+ Coverage    70.51%   70.76%   +0.24%     
===========================================
  Files          132      130       -2     
  Lines         8107     8079      -28     
  Branches       781      778       -3     
===========================================
  Hits          5717     5717              
+ Misses        2390     2362      -28     

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[simondsmart]

Trivial PR, assuming corresponding PR in multio.