Solve oringinal GemPy strata order issue

[GeorgeLiang3]

Problem: In Original GemPy, different grids configuration are used to compute the geological model and gravity. More specifically, the regular grid is used for geological models, and the centered grid is used to calculate gravity. The two grids are generated differently. The regular grid has the origin (0,0,0) at the lower corner, while the centered grid has the origin at the upper corner. This makes the order of the cell index in two directions. For regular grid, it's increasing upwards; for centered grid, it's increasing downward. This is not an issue of we deal with them separately, like what we have in the original GemPy. However, it becomes a problem if we would like to map the lithologies and the densities to the grid at the same time. It has the lithology and the densities in the wrong order. With the addition of the 'slope' technique, it is even harder to detect the correct value.
For example, a three layer model has lithology index (1,2,3) and densities values(4,5,6). In the original gempy, if mapping the two values to the same grid without care, it will be something like (1,2,3,6,-1,3)