code: public RegionMap toSubRegion( Envelope envelope ) {
          double w = envelope.getMinX();
          double s = envelope.getMinY();
          double e = envelope.getMaxX();
          double n = envelope.getMaxY();
          return toSubRegion(n, s, w, e);
      }
docs: Creates a new {@link RegionMap} cropped on the new bounds and snapped on the original grid.
  
  <p><b>The supplied bounds are contained in the resulting region.</b></p>
  
  @param envelope the envelope to snap.
  @return the new {@link RegionMap}.