Add LPM effect for photopair, fixes for BremsLPM

[Jean1995]

New PR for #334 but with clean history.

This PR adds the LPM effect for pair production of electron-positron pairs by photons.

Furthermore, this PR fixes the following issues in the implementation of the LPM effect in Bremsstrahlung:

1. An inconsistent definition of eLPM has been fixed
2. A more general definition of s1 is now used, to that the BremsLPM parametrization is not only valid for muons, but also for other particles (electrons/positrons/taus)

[Jean1995]

Enclosed are some exemplary comparisons of Bremsstrahlung dEdx between this PR and the current master:

[sudojan]

do we really need this constructor with mol_density, mass_density, sum_charge, eLPM? Some variables are not really intuitive.

[Jean1995]

eLPM is a computationally expensive calculation that needs to be executed only once, so we should save it.
mol_density_ and sum_charge_ are medium variables. Since those are the only two variables we need for the calcluations, we save them to avoid saving the whole medium in the PhotoPairLPM object. So I believe these three values are useful to save. Their names are just adopted from what they are called in the Medium class (although I agree that the variable names are not always well-chosen, but for the sake of consistency, I would use the same names here).

mass_density_ is only used in the constructor, so I would agree that we don't need to save them as a private member of the object. But in my opinion, this is the only change I would make to the constructor.

[Jean1995]

As an additional validiation, I have created plots where I compare the new dEdx results to the results from Polityko et al:

Note that the energies used in their paper only go up to 1e18 MeV, but it is sufficient to see that the agreement is now much better.