Theory citations improvements

[MJKirk]

This is mostly a record of theory citations I know/ believe to be missing from flavio right now (as taken from my comment in the middle of the pull request: #123 (comment))

1. ee->WW scattering
   

   flavio/flavio/physics/scattering/ee_ww.py

   Line 1 in 201a4aa

   r"""Functions for $e^+ e^-\to W^+ W^- scattering"""

   
   All the specific numbers in the formulae suggest to me they were pulled from somewhere.

2. Neutrino trident
   

   flavio/flavio/physics/neutrinos/trident.py

   Line 33 in 201a4aa

   sigma_tot = abs(CA_m)**2 + abs(CV_m)**2 + abs(CA_NP_e)**2 + abs(CV_NP_e)**2 + abs(CA_NP_t)**2 + abs(CV_NP_t)**2

   
   This general expression isn't in the "original" nu trident paper 1406.2332, and it's not obvious to me that everything else drops out such that you only need the wilson coefficients. I'm not sure if this expression was taken from a paper somewhere, or maybe it is sufficiently simple with a bit of thought. 🤔
   This is just a general effective theory limit of what's in 1406.2332, so a flavio.citations.register("Altmannshofer:2014pba") can be added.

3. General B decay stuff
   I did my best, but I think it would be worth someone with a better knowledge of the B decay literature than me checking everything in the bdecay folder to see if I've missed things.

[MJKirk]

Yeah okay, I've sat and thought about it and I see it now.

[peterstangl]

3. General B decay stuff
   I did my best, but I think it would be worth someone with a better knowledge of the B decay literature than me checking everything in the `bdecay` folder to see if I've missed things.

I noticed that table 1 in https://arxiv.org/abs/1810.08132 actually contains a quite complete list of references for the "general B decay stuff".

In addition, there are also the references from table 2 of this paper that we might want to add somewhere in the code.

But we might have to think about footnote 1:
"Note that this is not necessarily the reference to the first discussion of a particular observable in the literature but in some cases a review article. For original references, please consult reviews of the field, e.g. [34, 35]."