ENH: Add original I,J,K as properties in NestedHybridGrid

[nourinmohd]

Resolves #470

• Adds preservation of original grid I/J/K coordinates for NestedHybridGrid.

• _build_nested_hybrid_grid() now appends I_orig, J_orig, and K_orig properties to the coarse grid before refinement and merge, allowing xtgeo.grid_merge() to transfer them automatically to the final nested hybrid grid. These properties are available through nhg.properties and nhg.grid.get_prop_by_name(...).

• Also updates the nested hybrid grid property test to verify the new properties are present and retain the original 1-based coordinates.

Checklist

• Tests added (if not, comment why)
• Test coverage equal or up from main (run pytest with --cov=src/ --cov-report term-missing)
• If not squash merging, every commit passes tests
• Appropriate commit prefix and precise commit message used
• All debug prints and unnecessary comments removed
• Docstrings are correct and updated
• Documentation is updated, if necessary
• Latest main rebased/merged into branch
• Added comments on this PR where appropriate to help reviewers
• Moved issue status on project board
• Checked the boxes in this checklist ✅

[nourinmohd]

• Add a constant for the I_org, J_org, and K_org instead of passing them as strings.

[alifbe]

These properties is very useful for mapping between nested grid and parent grid which is hopefully can be fully automated from within NHG class. So, at least for now, we don't see the needs for this property to be exported to RMS. Please ensure that .to_rms() method doesn't export these IJK properties.

[nourinmohd]

Implemented:

• Renamed IJK properties to parent_I, parent_J, and parent_K.
• .to_rms() now excludes these internal mapping properties.
• Updated tests to use ORIGINAL_IJK_PROPERTY_NAMES.

[tnatt]

This looks good to me 🙂
We might want to add some info regarding these properties into the documentation, to inform they are there.. but we can maybe wait and see if the need arises. If there is a need, the best solution would perhaps be to add a flag in the to_rms function to easily store these properties in rms.

[alifbe]

Looks great. Please consider changing the commit message. For some case where user do nested over nested, original could be interpreted as origin grid. But in this case, the property that is created is direct parent.