Create recipes

[alecandido]

• create recipe files in a new EKO
  • new EKO: empty recipes/, parts/, and operators/ folders
• double check the information in the recipe to be sufficient for computation

[alecandido]

In commit 85975ad a further breaking change is introduced.

There is no hurry to merge this PR, for as long as there is not the full alternative runner. In any case, even the full alternative runner will deprecate, but not immediately drop, the old one, so this will still be compatible.

Since I had to play with these quantities to use them in the new runner as well, I decided to start improving (or just factorizing) a bit the content of the current one.
In light of #205 I tried to improve the existing classes.

However, if the changes are welcome, I can keep going fixing the tests/benchmarks/docs (in principle should simplify the current definitions, also clarifying the role of linear and squared scales).
Otherwise, it is sufficient to drop that commit. @felixhekhorn let me know.

[alecandido]

This is now rebased on top of #220, and I will restart work on this one as soon as #220 will be close to final.

[alecandido]

By now, this PR has a wrong name (and the board is even messy). The goal is to have a minimal jets runner, otherwise it has hard to test and compare against the previous one.

I opened #242 to replace this.