Remove ev_op_iterations from truncated

[felixhekhorn]

Closes #447

As a consequence ev_op_iterations also dropped from kernels.ns.dispatcher, i.e. in the non-singlet sector there is no iteration anywhere (since exact is exact). This triggers a few more changes on the testing side.

[giacomomagni]

This looks fine for me, the only other option I see is to hardcode the default value to 1 in the function signature. But also dropping it's fine!

[felixhekhorn]

This looks fine for me, the only other option I see is to hardcode the default value to 1 in the function signature. But also dropping it's fine!

I would prefer to remove it completely to also not confuse developers 🙃 - if you're happy please approve 😇

Since I just told our PhD students here how important documentation and as a part of that a Changelog is I finally started our Changelog here 🙈 thus: Closes #216

Note that in 0.15 there are so many changes that it was impossible for me to spell it out quickly (remember that all the long-standing PRs from back in the days (e.g. #295) got merged there)