Spectrum plotting should not break when loading generic spectral data

[havok2063]

Describe the bug Loading and plotting generic spectral data into Spectrum does not work. I tried to create a spectrum out of an HI spectral data (flux and velocity). The spectrum is created but plotting fails because it either looks for a pix_mask or requires std error array data. Still fails when either use_mask or show_std is set to False.

To Reproduce

import astropy.units as u
from marvin.tools.quantities.spectrum import Spectrum

# create a generic spectrum
s=Spectrum(hi_data._specdata['FHI'][0], wavelength=hi_data._specdata['VHI'][0], scale=1, unit=u.Jy, wavelength_unit=(u.km/u.s))

# try to plot it
s.plot()
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-63-886abc61b753> in <module>()
----> 1 s.plot()

/Users/Brian/Work/github_projects/Marvin/python/marvin/tools/quantities/spectrum.pyc in plot(self, xlim, ylim, show_std, use_mask, n_sigma, xlabel, ylabel, show_units, plt_style, figure, return_figure, **kwargs)
    191
    192         if use_mask:
--> 193             donotuse_mask = self.pixmask.get_mask('DONOTUSE') > 0
    194
    195         if use_mask:

/Users/Brian/Work/github_projects/Marvin/python/marvin/tools/quantities/base_quantity.pyc in pixmask(self)
    120         """
    121
--> 122         assert self.pixmask_flag, 'pixmask flag not set'
    123
    124         pixmask = maskbit.Maskbit(self.pixmask_flag)

AssertionError: pixmask flag not set
Init signature: specutils.Spectrum1D(self, flux, wcs, unit=None, uncertainty=None, mask=None, meta=None, indexer=None, *args, **kwargs)
Docstring:
A subclass of `NDData` for a one dimensional spectrum in Astropy.

# try to plot it with no mask
s.plot(use_mask=False)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-64-48cc84529cc3> in <module>()
----> 1 s.plot(use_mask=False)

/Users/Brian/Work/github_projects/Marvin/python/marvin/tools/quantities/spectrum.pyc in plot(self, xlim, ylim, show_std, use_mask, n_sigma, xlabel, ylabel, show_units, plt_style, figure, return_figure, **kwargs)
    200             value = self.value
    201             wave = self.wavelength.value
--> 202             std = np.ma.array(self.std.value, mask=donotuse_mask)
    203
    204         ax.plot(wave, value, **kwargs)

AttributeError: 'NoneType' object has no attribute 'value'

Expected behavior At minimum, it should be able to take a flux array, some x-axis data and plot it.

Additional Context Same problem occurs when trying to plot random data against an integer range.

[bretthandrews]

Fixed in vetround2 branch