Nested hybrid grid: import as a reconstructed LGR hierarchy

[kriben]

Add support for importing Eclipse "nested hybrid grids" (produced by xtgeo/fmu-tools). Such a grid is a single flat EGRID in which the refined region is appended to the end of the I axis at higher resolution and connected to the coarse grid through NNCs; in IJK space the refined cells are far from their coarse parent, but their COORD/ZCORN place them inside the coarse region. The coarse parent cells are collapsed to zero volume, so the parent of each refined cell is supplied explicitly by a HOSTNUM sidecar.

Both sidecars are auto-detected next to the grid file:

• _NEST_ID.grdecl : per-cell nesting level, loaded as a discrete category property for coloring/filtering.
• _HOSTNUM.grdecl : per refined cell, the 1-based natural index of its parent coarse cell (0 otherwise).

RigNestedHybridGridReconstructor rebuilds each refined region as a true RigLocalGrid: it appends contiguous cells/nodes, copies the real refined geometry, links each cell to its parent (subGrid/parentCellIndex), hides the original scattered cells, transfers active-cell result indexing, copies result values onto the LGR cells, and re-points the file NNCs to the LGR cells. The reconstructed LGR is flagged so it is excluded from the on-file grid count, preventing the result reader from reading it as a phantom grid.

The refined region then appears as a proper LGR with correct geometry, INDEX_I/J/K, static/dynamic results and computed results (e.g. SOIL). Normal grids (no sidecar present) are unaffected.

Key changes:

• RigNestedHybridGridReconstructor (new) + RimEclipseResultCase hooks
• RigLocalGrid isReconstructedGrid flag; RigMainGrid gridCountOnFile and nestedHybridLgrSourceCells
• RigCaseCellResultsData: copy results onto reconstructed-LGR cells
• RiaResultNames: NEST_ID name + category-result handling
• RigActiveCellInfo::computeDerivedData made idempotent
• Unit tests in RigNestedHybridGridReconstructor-Test.cpp