d3q Code

Looks like my brilliant idea was not that brillians, fixing previous commit, now two matrices are used again (but not for SMA) also looks stable but I have to test

New form for variational tk (as in PRB paper) seems to work much better than the previous one (that was from the thermalk code), it is also twice as fast as the same matrix can be reused, but some more testing is needed. In particular, I have some doubt about the index to use in freqm1

recompute-sma modified to use only files from tk.x with store_lw=.true.: using hand-made lineiwidths is too complicated and you'ld have to do it by hand anyway
also, recompute-sma-mfp shows how to recompute sma with a cut-off on the mean free path

I did not put the 0.5 factor after all..

Implemented a smart mechanism to stop the code immediately if the step end is after the wall limit from input, to save CPU time

Add the possibility of doing a SMA tk calculation without the intrinsic ph-ph scattering, in order to produce the isotopic and casimir linewidth to combine using recompute-sma.m

Many little changes here and there,
most notably, the neutron form factor is more or less reliable (although it only includes Bi Se and Te elements)

Improvements in the Neutron scattering code, inclusion of elastic peak (its norm has to be specified), convolution with experimental width now optional (set to zero to disable it)

Error message clarified

Code taken to level with git version of QE