Document gridding's data characteristics

[scpmw]

The way gridding works, we have a few connected characteristics of the handled data:

• In doimg, we "mirror" coordinates and conjugate visibilities. In my understanding, this is correct because it reflects "inverted" baselines.
• This causes in turn causes the result of gridding to be hermitian
• This property is what makes sure that the result image of the FFT is, in fact, real-valued

It would make the program easier to follow if this was spelled out explicitly.