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.