Update WCS in spectral_slab output

While spectral slab does output the correct sub-wavelength range as a new Spectrum1D, it does not appear to update the WCS to match. This means going through a lot of trouble to update the wcs manually, like this:
```python
import astropy.wcs as fitswcs
from specutils.manipulation import spectral_slab

my_wcs = fitswcs.WCS(header_cube)
spec1d = Spectrum1D(flux = cube*u.Jy, wcs=my_wcs)
trim=[1.57E-6*u.m, 1.62E-6*u.m]
spec1d_t = spectral_slab(spec1d, trim[0], trim[1])
cube_trimmed = spec1d_t.flux.value

#Adjust wcs to trimmed cube because spectral_slab doesn't
new_wcs = spec1d_t.wcs
new_wcs.wcs.crval[0] = trim[0].value #Change wave crval to match trim region
print(new_wcs)

spec1d_trimmed = Spectrum1D(flux=cube_trimmed * u.Jy, wcs=new_wcs)
```

Start at the spectral_slab entry point and reproduce the supplied Spectrum1D example with the stated wavelength limits. Compare the returned Spectrum1D WCS with the trimmed spectral range; done means the output WCS matches that range without requiring the manual adjustment shown.

Written by the indexing model from the issue text.