Visibility weights and dead dipoles

[cjordan]

The weights should probably be multiplied by the fraction of dipoles that are alive.

e.g. tile 1 has 15/16 X dipoles alive, tile 2 has 16/16 X dipoles alive. The weight for the XX visibility formed on the baseline tile1-tile2 should be multiplied by 15*16 / (16*16) (or something similar to this). XY would depend on tile 2's alive Y dipoles etc.