Reuse ekos for nFONLL

[felixhekhorn]

FFNS and FFN0 are the same from the evolution point of view so there is no need to compute the same eko twice, we can just inherit

[giacomomagni]

Now that we have fixed the number of theory to be always 7, we should implement 2 commands doing:

The first one (nfonll_ekos):

1. generating the 3 operator cards
2. computing all the ekos in the 3 flavor patches
3. inheriting the ekos

And a second one (nfonll_fks):

1. computing the various fktables
2. combining the fktables

Both command should work for multiple datasets.
This way the user will not have to loop manually on the nFONLL theories which is really error prone.

We can also think of the "one command" automation to get straight the fk-table, but that will not be useful for Sv, for which you don't want to recompute the eko each time...