iData_vDOS: create a vibrational density of states data set (iData flavour)
 
  The iData_vDOS class is a 1D data set holding a density of states
    often labelled as g(w). This usually can store a 'true' vibrational density 
    of states (vDOS), or a generalised density of states (gDOS).
 
  Example: g=iData_vDOS('SQW_coh_lGe.nc')
 
  Useful methods for this iData flavour:
 
  methods(iData_vDOS)
    all iData methods can be used.
  iData_vDOS(g)
    convert input [e.g. a 2D or 4D Sqw, or a 1D vDOS] into an iData_vDOS to give access to
    the methods below.
 
  d = dos(g)
    Get the vibrational density of states (vDOS).
 
  t = thermochemistry(g)
    Compute and display thermochemistry quantities from the vDOS.
 
  [Sqw,Iqt,DW] = incoherent(g, q, T, sigma, m, n, DW)
    Compute the incoherent gaussian approximation scattering law S(q,w) from an initial 
    density of states vDOS.
 
  [Gw, Tsym] = multi_phonons(g, Ki, T, sigma, m, phi, n)
    compute the integrated multi-phonon generalised density of states (gDOS) 
    from an initial vibrational density of states (vDOS) in the neutron 
    scattering incoherent gaussian approximation.
    The generalised density of states (gDOS) is the sum of the terms in this expansion.
 
  gDOS = gdos(vDOS, Ki, T, sigma, m, phi, n)
    compute the generalised density of states (gDOS) from a vibrational 
    density of states (vDOS) in the neutron scattering incoherent gaussian approximation.
 
  vDOS = vdos(gDOS, Ki, T, sigma, m, phi, n)
    compute the 'true' vibrational density of states from a gDOS estimate
    in the neutron scattering incoherent gaussian approximation.
 
  syntax:
    g = iData_vDOS(s)
  input:
    s: can be an iData (1D vDOS, 2D Sqw, 4D Sqw) or filename to generate a 1D vDOS object.
 
  output: an iData_vDOS object