Issues in edge of speedup-LBL

[riclarsson]

Is your enhancement request related to a problem? Please describe.
propmat_clearskyAddLines is wrong when speed-up is used around the upper range as seen in this plot:

Describe the solution you'd like
The relative errors should be relatively constant at all frequencies.

Describe alternatives you've considered
I've looked at the code and I cannot figure it out. The alternative is to run with a wider frequency grid than desired and throw-away the tail for any real computations.

Are you willing to help adding this enhancement?
Not more. I have tried. I can provide test-files on request.