qe-tools: handle transition moments output from bands.x

[ddkohler]

https://www.quantum-espresso.org/Doc/INPUT_BANDS.html - see lp, flip

for format, here is an excerpt from graphene output

&p_mat nbnd=   8, nks=  48 /
            0.000000  0.000000  0.000000      4
  1
     0.00000000     0.00000000     0.00000000     0.00000000
     0.00000000     0.00000000     0.00000000     0.00000000
     0.00000000     0.00000000     0.00000000     0.00000000
     0.00000003     0.00000000     0.50951384     0.00000837
  2
     0.00000000     0.00000000     0.00000000     0.00000000
     0.00000000     0.00000000     0.00000000     0.00000000
     0.00000000     0.00000000     0.00000000     0.00000000
     0.00153141     0.00000000     0.00000837     0.50951393
  3
     0.00000000     0.11774035     0.00000000     0.00000000
     0.00197918     0.00000000     0.00000000     0.00000000
     0.00000000     0.18850589     0.00000000     0.00000000
     0.00000000     0.00000000     0.00000000     0.00000000
            0.016667  0.028868  0.000000      4
[continues for all kpoints]

The labels of each 4x4 matrix refers to the respective cartesian coordinate. The elements of each 4x4 matrix is m[i,j] = VB[i]--> CB[j] (there are 4 VBs and 4 CBs).

Not currently sure of the assignments of the VB, CB. Cannot find documentation on file output, but can be inferred: https://gitlab.com/QEF/q-e/-/blob/develop/PP/src/write_p_avg.f90

No clue how stable this output file format is...