iFit/sqw_thermochemistry

 sqw_thermochemistry: compute thermodynamic quantities.

 compute thermodynamics quantities.
 When missing, the phonon (vibrational) density of states (vDOS) from e.g. the 
 evaluation of the 4D sqw_phonons object.

 The input 4D model should be e.g. a 4D phonon one, with axes qh,qk,ql,energy
 in [rlu^3,meV]. The evaluation of the model should provide the density of states
 in model.UserData.DOS as an iData object.

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

 input:
   s: a 4D sqw_phonons model [K,H,L,Energy] (iFunc)
   T: temperature range. When not given, T=1:500 [K]
   options: can be 'plot' to display the results.

 output:
   t: structure with [DOS, DOS_partials, S U F Cv ]
   fig: figure handle