Hex Map 23 Fix

A great patron just made we aware of a bug introduced in Hex Map 23. I omitted adjusting the saving and loading code to deal with negative elevation. The result is that cells with negative elevation get stored with very high elevation instead.

I have added section 4.4 Storing Negative Elevation to Hex Map 23. Give it a look. I've chosen to offset elevation by 127 in the save data. You could also just use an integer, requiring three extra bytes per cell.