Compute all pending ensemble contour maps in one sweep over realizations

[magnesj]

When several ensemble contour maps are pending computation, they are now computed in a single sweep over the ensemble realizations, so that each realization case is opened once instead of once per contour map. ensureResultsComputed() collects the pending sibling maps with views and passes them to the new computeStatisticsForMaps(), which merges the previous shared-grid and individual-grid code paths.

Additional fixes discovered while testing:

• A sibling map can be computed before its own initAfterRead() has run, and then the result definition has no case to resolve the result address against, producing empty results. The result definition case is now initialized before computation.
• The contour map grid bounding box was computed before the primary case was opened, producing an empty 0-cell grid if the case was not already open. The primary case is now opened first.
• The overlay info statistics for contour map views always showed "Sample Count: 0" since the histogram vector was never populated. The histogram is now computed from the map projection values, skipping invalid cells. This applies to both Eclipse and GeoMech contour map views.

Fixes #14319