Add (and use) a default for IterEv
Created by: scarlehoff
I wanted to deal with this comment by @felixhekhorn https://github.com/NNPDF/nnpdf/pull/2028#discussion_r1574778983 and it ended up growing so much that I thought it would be better to separate it in its own PR so it can be reviewed before merging it to #2028.
This does very little:
- Adds some conditional default (and tests for them)
- Ensures that
evolven3fit
will make use of that default and, in addition, will loudly fail if someone tries to mix stuff.