spectral extraction: improve background defaults for spectrum near top/bottom

[Jdaviz-Triage-Bot]

Reporter: Kyle Conroy

if the spectrum is near the top or the bottom, the background region can go out of bounds by default (resulting in a failed extraction) or would squeeze too tight to the extraction trace.  This logic should be improved to have the default switch to one-sided (in the appropriate direction), when appropriate.

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DISCLAIMER: This issue was autocreated by the Jdaviz Issue Creation Bot on behalf of the reporter. If any information is incorrect, please contact Duy Nguyen