Extend Trace/Background/Extract to support specutils.Spectrum1D objects as inputs

[kecnry]

this would be useful, especially for tracking units and so that wavelength solutions can be propagated to the returned objects.

[pllim]

Back in spacetelescope/jdaviz#1608 , I tried doing this but it turned flux unit from MJy into MJy2. But without this (current behavior), it turns MJy into DN.

--- a/jdaviz/configs/specviz2d/plugins/spectral_extraction/spectral_extraction.py
+++ b/jdaviz/configs/specviz2d/plugins/spectral_extraction/spectral_extraction.py
@@ -644,7 +644,9 @@ class SpectralExtraction(PluginTemplateMixin):
             self.update_marks(step='ext')

         trace = self._get_ext_trace()
-        inp_sp2d = self._get_ext_input_spectrum()
+
+        # FIXME: Passing in Quantity turns MJy into MJy2. But not passing it in gives DN.
+        inp_sp2d = self._get_ext_input_spectrum().flux

         if self.ext_type_selected == 'Boxcar':
             ext = extract.BoxcarExtract(inp_sp2d.data, trace, width=self.ext_width)