Add new switch to subcommand `channels` to show separate entries

[cschwan]

When using pineappl channels [INPUT] [PDFSET] for a grid with the following lumi (DY LO):

l    entry        entry        entry
-+------------+------------+------------
0 1 × ( 2, -2) 1 × ( 4, -4) 
1 1 × (22, 22)              
2 1 × ( 1, -1) 1 × ( 3, -3) 1 × ( 5, -5)

it is very useful to also tell apart the different entries for each luminosity function, for instance the size of the up-anti-up channel separate from the charm-anti-charm channel.

[cschwan]

One way to implement this feature would be #165.

[cschwan]

I will not implement this, because it can be worked around using pineappl write --split-lumi, see commit a9a99af1f100b77dc0db5583cc5884ddf5bc2124.