allow to register agents after activation of container

[maurerle]

this makes sure that the agent._do_start and agent.on_ready functions are called as expected

this also fixes a bug in runtime management of the external_coupling container, which did set running before starting the container.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.69%. Comparing base (b0c1850) to head (b0c8a1d).
Report is 2 commits behind head on development.

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #132      +/-   ##
===============================================
+ Coverage        88.65%   88.69%   +0.03%     
===============================================
  Files               22       22              
  Lines             2407     2415       +8     
===============================================
+ Hits              2134     2142       +8     
  Misses             273      273              

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[rcschrg]

Actually, I also implemented this on another branch and just forgot to merge it. However, your version is actually better :)