mo_grid_scaler: add median upscaling operator

This MR adds a new up_median upscaling operator to mo_grid_scaler.

The operator computes the median of the fine-cell values covered by a coarse cell and is available only for floating-point output. It supports both dp -> dp and i4 -> dp execution paths, while integer-output median upscaling remains unsupported by design. The implementation uses mo_orderpack::omedian and relies on the existing grid coverage checks, so no special handling for empty coarse cells is added.