S1 pulse shape debug

[zhut19]

What is the problem / what does the code in this PR do
The S1 photon timing from nestpy is not completely shuffled, the first n_exiton entries are just from primary excitons and the rest from recombination photons. See,
https://github.com/NESTCollaboration/nestpy/blob/fe3d5d7da5d9b33ac56fbea519e02ef55152bc1d/src/nestpy/NEST.cpp#L164-L179

Note: https://xe1t-wiki.lngs.infn.it/doku.php?id=xenon:j_angevaare:wfsim_photon_timing_bug

WFSim needs you:

• Please add a test for this PR, as a bare minimum, make sure it's covered in coveralls!
• Can you add a docsting to all your functions?

Pay attention:

• Due to databases being needed for testing, making a PR from your own fork will typically NOT run the tests. If you then merge master might break

[coveralls]

Coverage decreased (-0.03%) to 82.364% when pulling d1dcc9b on zhut19-patch-1 into c34ba24 on master.

[ramirezdiego]

Linking this baby here: https://github.com/XENONnT/private_nt_aux_files/pull/202