Half-pixel mismatch with original Foster Vs30 map

[AndrewRidden-Harper]

@elshawy has identified that the map produced by this Python code is offset by half a pixel from the original Foster Vs30 map. This is probably due to the map generation codes using different implicit definitions for which part of the pixel corresponds to the coordinate. For example, one codebase might assume that the coordinates correspond to the middle of each pixel, while the other assumes that the coordinates correspond to one of the corners of each pixel. To resolve this issue, we should decide which coordinate to pixel definition to use. Note that this is not related to the significant refactoring of the Vs30 codebase, as the pre-refactored and refactored versions have equivalent output.