v1.18.0

New Features

• New Spectrum1D.with_spectral_axis_and_flux_units method to convert both
  spectral axis and flux units at the same time. [#1184]

Bug Fixes

• Fixed Spectrum1D.with_flux_unit() not converting uncertainty along
  with flux unit. [#1181]

• Fixed extracting a spectral region when one of spectrum/region is in wavelength
  and the other is in frequency units. [#1187]

Other Changes and Additions

• Replaced LevMarLSQFitter with TRFLSQFitter as the former is no longer
  recommended by astropy. [#1180]