@classmethod
def from_center(cls, center=None, width=None):
    """
        SpectralRegion class method that enables the definition of a `SpectralRegion`
        from the center and width rather than lower and upper bounds.

        Parameters
        ----------

        center : Scalar `~astropy.units.Quantity` with pixel or any valid ``spectral_axis`` unit
           The center of the spectral region.

        width : Scalar `~astropy.units.Quantity` with pixel or any valid ``spectral_axis`` unit
           The width of the spectral region.
        """
    if width.value <= 0:
        raise ValueError('SpectralRegion width must be positive.')
    return cls(center - width, center + width)
