Add support for additional couplings

[cschwan]

For effective field theories it would be desirable to support additional couplings, see for example here: https://answers.launchpad.net/mg5amcnlo/+question/699629. Without it PineAPPL adds all contributions that have the same coupling in alpha and alphas together.

Technically this requires changing the Order struct, which we can do in a new file format, which requires #83 to be merged.