Add primary beam correction to 2D images

[mtazzari]

At the moment, the input image in *Image() and the images internally created in *Profile() are not primary beam (PB) corrected. Namely, they are assumed to be already PB-corrected.

It would be good to implement PB correction. I am currently evaluating two possible ways:

1. either take in input the antenna Diameter and assume a functional form, e.g. J_1(pi D u)/u with J_1 the modified Bessel function, D the antenna diameter and u the u-coordinate.

2. or take in input a 2D matrix containing the PB image (e.g. the one produced by CASA clean).

If we can assume that the ALMA PB is generally well behaved, then (1) should be more convenient computationally: since it would be axisymmetric, it would just be a function multiplying the profile in *Profile(). For *Image() we would need to multiply the whole image anyway.