Draft: Updates to Allen-Heine-Cardona calculation

The only change to the non-AHC part is the initialization of image parallelization.

Currently, image parallelization distributes workload considering the symmetry of q points, which is useful for DFPT. But, in el-ph-only calculations (trans=.false.), workload is the same for all q points. Hence, I added an option assume_equal_load to the subroutine image_q_irr.

Changes on the AHC part

• Allow using irreducible q grid in AHC calculation: need use_irr_q = .true. in postahc.x input.
• Reduce omega_zero_cutoff
• Read xml file in postahc.x
• q point parallelization in postahc.x
• Add AHC window to postahc.f90. New input parameters: ahc_win_min, ahc_win_max
• Allow truncation of Debye-Waller self-energy. Replace skip_upperfan in previous versions by skip_upper.
• Update postahc.x test-suite.
  • Output unit changed from Ry to eV.
  • nbnd for nscf changed from 12 to 9 to avoid gauge dependence (band 9 and 10 are always nondegenerate, while band 12 and 13 is degenerate at some k)