iFit/thermochemistry

 iData_Sqw2D/thermochemistry: compute thermodynamic quantities for 2D S(q,w) data sets.

 Compute thermodynamics quantities.
 When missing, the generalised density of states (gDOS) is estimated 
 from the 2D sqw object.

 The input 2D data set should be e.g. a 2D S(q,w), with axes |q|,energy
 in [Angs-1,meV]. 

 the function returns an array of iData objects:
   DOS                                 [modes/energy unit/unit cell]
   entropy                           S [eV/K/cell]
   internal_energy                   U [eV/cell]
   helmholtz_energy                  F [eV/cell]
   heat_capacity at constant volume Cv [eV/K/cell]

 Reference: https://wiki.fysik.dtu.dk/ase/ase/thermochemistry/thermochemistry.html#background
            D.A. McQuarrie. Statistical Mechanics. University Science Books, 2000.

 syntax:
   t = thermochemistry(s)
   t = thermochemistry(s, Tmin:Tmax)

 input:
   s: a 2D sqw data set [|Q|,Energy] (iData_Sqw2D)
   T: temperature range. When not given, T=1:500 [K]
   options: can be 'plot' to display the results.
         The 'newplot' option does the same but opens a new figure instead of
         re-using an existing one.

 output:
   t:   iData array with [DOS S U F Cv ]
   fig: figure handle

 See also: iData_Sqw2D/dos, iData_vDOS