Add script to calculate mean, stdev for continuous 3D field parameter and facies probabilities for 3D facies parameter per zone

[oddvarlia]

…s and estimated facies probabilities for discrete 3D parameter

[oddvarlia]

Purpose:
Calculate ensemble mean and standard deviation for 3D continuous parameters and estimates for facies probabilties for 3D facies parameters.
The estimates should be available per zone in the geomodel grid also for ensembles where the 3D geomodel grid vary from realization to realization as long as number of grid cells laterally (nx, ny) is fixed. To handle this, a fixed "gox" grid called ERTBOX grid is used.
The plan is also to be able to calculate this for field parameters used in APS in AHM workflows in ERT using ES-MDA:

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 88 lines in your changes missing coverage. Please review.

Project coverage is 85.43%. Comparing base (d07ca3f) to head (30103d7).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/subscript/field_statistics/field_statistics.py	75.00%	88 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #739      +/-   ##
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- Coverage   85.97%   85.43%   -0.54%     
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  Files          51       52       +1     
  Lines        6837     7189     +352     
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+ Hits         5878     6142     +264     
- Misses        959     1047      +88     

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[oddvarlia]

I will add a specification using yaml as config file format instead of directly specifying a python dict.

[alifbe]

I think this PR looks great. I am still not sure if you need to have ert/model in your testdata folder.

Perhaps @rnyb or @mferrera can also have a look at this PR?

[mferrera]

There are a few improvements I think are possible here (print statements, commented out code, etc) but as a first pass to initialize a script I think we can take this as is for now.