Request for new features of rtabtrans1+rtabtrans2

[jongrumer]

1. We should add the possibility to print level energies in the line list produced by the ratbtrans1 + rtabtrans2 codes - for these codes it would also be good to have the possibility to print the lifetimes of the levels in the list, in order to estimate the radiative broadening of the level.

   Connected to PR #85 .

2. We should also have a clean way of rescaling the transition energies to experimental energy levels. I have a hacked way of doing this by adding experimental/NIST energies to a level list produced by rlevels, which is then read and rescaled by rtabtrans1. But it's not really ready to be submitted. Maybe someone else have a more production ready version? Or a more clever technique? I think perhaps @WenxianLi might have some code that does this? :)

/Jon