Replace trap.id by trap.name and convert id argument to attribute

[KulaginVladimir]

oh, yes. I just meant that, personally, I find convinient to define export for (and then access) traps with their ids. Though it might be a matter of taste.

Right, so i think we need to make a different between the index of the traps in the list (only used for constructing the function space) and the tag/name of the trap that is a user-friendly way of referring to it.

We can have users refer to a trap in for example exports by either using the actual trap object, or its name, or its index in the trap list.
We remove the id as a user-defined argument and just have it for convenience as an attribute (maybe).
Then we simply have to forbid users to use digits-only names.

Anyway we can merge this PR and ship the discussion elswehere

Originally posted by @RemDelaporteMathurin in #810 (comment)