mosaicing at low declination

[twshimwell]

Presently at low dec we use an elliptical beam which is a bit of a pain when mosaicing because of projection effects meaning its slightly rotated between all the pointings we are mosaicing.

Could either:

• switch to a circular beam for the low declination obs for simplicity
• during the mosaicing calculate the smallest ellipse the encompasses all the individual restoring beams (once projected onto a common projection).

[twshimwell]

After discussion with @mhardcastle we shall take the approach of convolving to a circular beam during the mosaicing where we use the largest bmaj of all pointings being mosaiced together.

[twshimwell]

Now issue described in ticket #354 so this one can be closed.