cannot emulate gbtidl's edge smoothing

[teuben]

Describe the bug
The default smoothing near the edge of a spectrum uses /edge_truncate ig gbtidl, which is the same as boundary="extend", but specutils smoothing has no API to use **kwargs to override the boundary='fill' default.
Until specutils has this API change (requested), we cannot exactly reproduce GBTIDL smoothing. For good measure the test should pick some central portion of the spectrum.

How to Reproduce

Environment

• Dysh version
• Python version
• OS

[teuben]

I've submitted a discussion issue on specutils astropy/specutils#1147

[teuben]

Since I switched to core.smooth where astropy's convolution is used, this issues is less of an issue for us. I'm thus closing it for now,but keep the one in specutils open.