Separate multigrid watcher setup endpoint from watcher start endpoint

[d-j-hatton]

Allows better control over when watching starts so that early starting (i.e. before processing parameter specification) can be avoided

[codecov]

Codecov Report

Attention: Patch coverage is 17.64706% with 14 lines in your changes missing coverage. Please review.

Project coverage is 28.22%. Comparing base (8567d8d) to head (07d50a0).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #566      +/-   ##
==========================================
- Coverage   28.78%   28.22%   -0.57%     
==========================================
  Files          80       80              
  Lines       10605    11021     +416     
  Branches     1414     1586     +172     
==========================================
+ Hits         3053     3111      +58     
- Misses       7450     7799     +349     
- Partials      102      111       +9     

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