Meaning of f_min in the SpectralInformation

[jktjkt]

Right now in the JSON format, the first carrier is placed at f_min + spacing. That's not intuitive because users would typically expect them to start at f_min as the first central frequency (or, possibly, as the lowest passband edge frequency as the docs mention the edge at some places). However, doing that in the existing legacy-JSON would lead to backward-incompatible change.

References:

• https://review.gerrithub.io/c/Telecominfraproject/oopt-gnpy/+/507044/comment/ec702729_30517a01/
• https://review.gerrithub.io/c/Telecominfraproject/oopt-gnpy/+/535245/comments/c81cf92d_ab0feba9

[EstherLerouzic]

maybe change the behaviour in a 3.x release also changing f_min to f_start (agreed during coders call)