Validate field parameter at startup

[oyvindeide]

Issue
Resolves #my_issue

Approach
Short description of the approach

Pre review checklist

• Added appropriate release note label
• PR title captures the intent of the changes, and is fitting for release notes.
• Commit history is consistent and clean, in line with the contribution guidelines.
• Updated documentation
• Ensured new behaviour is tested

Adding labels helps the maintainers when writing release notes. This is the list of release note labels.

[codecov-commenter]

Codecov Report

Merging #5869 (11727c0) into main (18ab918) will increase coverage by 0.05%.
Report is 9 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5869      +/-   ##
==========================================
+ Coverage   81.71%   81.76%   +0.05%     
==========================================
  Files         347      347              
  Lines       21718    21619      -99     
  Branches      739      737       -2     
==========================================
- Hits        17746    17676      -70     
+ Misses       3719     3690      -29     
  Partials      253      253              

Files Changed	Coverage Δ
src/ert/config/ensemble_config.py	95.05% <100.00%> (ø)

... and 31 files with indirect coverage changes

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