Fix primary beam correction in DS extraction

[joshoewahp]

The --primary-beam argument in dstools-extract-ds is supposed to generate a primary beam image when passed with a non-existing path, which is then sampled to generate a primary beam correction factor.

Currently the primary beam image generated in this way is always centred on the target coordinates rather than the antenna point centre, and therefore sampling the PB image produces a scale factor of 1.