Add an option in retile script to use multiple VRTs

[ramSeraph]

This will help reduce the amount of tiling needed when the changed sheets are too far apart.. The idea is to find clusters of sheets which affect common base tiles.

[ramSeraph]

I am no longer sure if this is required.. need to understand how GDAL picks which tiles to generate form a VRT to see if this is required. The pathological edge case that lead to the creation of this issue has since been attributed to other factors.

Keeping this open in case I run into it again.