pcotomo: replace usage of `nb_turn` + `tomo_n` by `tomo_n` + `nb frames`

It would improve robustness to avoid using nb_turn and only use the number of projections in the dataset.

Today we are splitting an NXtomo in n when we should split it from 0:tomo_n, tomo_n:tomo_n*2... So this would be a rewrite of the to_NXtomos_pcotomo_v2 function