KS_Solvers/CG, KS_Solvers/Davidson, KS_Solvers/Davidson_RCI.
Two are currently used by QE, the third one implements the Davidson
diagonalization within the Reverse Communication Interface paradigm,
courtesy of Micael Oliveira.

KS_Solvers routines depend only on lower level libraries, notably UtilXlib, 
LAXlib, (SCA)LAPACK, and BLAS.

reorganization can be improved. For instance some duplicated routines like
cdiaghg and rdiaghg could/should be moved in LAXlib. This could reduce the need
to include  KS_Solvers directories in the link step of many codes.    

Minimal changes to calling sequence have been made, essentially just adding
h_psi,s_psi,g_psi and h_1psi,s_1psi routines names as arguments (with a
specific calling sequence ihardcode inside the routines that agree with PWSCF one). 
This could be avoided adopting the RCI paradigm.

Compiled in serial and parallel, 177/182 pw tests passed (3 that were failing 
even before on my laptop pw-b...