add rporv workaround

[tralsos]

When RFIP for each phase is missing in the UNRST file, the preferred resdata gravimetry method add_survey_RFIP is failing. A workaround is to use the add_survey_RPORV method instead, which calculates RFIP from RPORV and phase densitities. This is currently needed when using OPM FLOW as simulator, since it does not output RFIP to the UNRST file. Solves #740

Adding this workaround allows for implementation of the gravity modelling in Drogon.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.92%. Comparing base (4209590) to head (438a6f7).

Files with missing lines	Patch %	Lines
src/subscript/grav_subs_maps/grav_subs_maps.py	50.00%	2 Missing ⚠️
src/subscript/grav_subs_points/grav_subs_points.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #743      +/-   ##
==========================================
- Coverage   85.97%   85.92%   -0.05%     
==========================================
  Files          51       51              
  Lines        6837     6843       +6     
==========================================
+ Hits         5878     5880       +2     
- Misses        959      963       +4     

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[alifbe]

Looks good to me 👍