Update HALO-20240921a.md: addition of flight categories

[raphvog]

We fly three circles, but all of them in the "middle" of the ITCZ. So I just put c_mid. I wonder whether we should add new categories for these last flights with c_west, c_mid and c_east? Could be discussed.

[JuliaWindmiller1987]

Thanks for adding these and yes, let's think about a way in which we can make the circles more meaningful!