Add phase integrals

[steindev]

Phase integrals are required to compute cross-sections in background fields. Integrals are defined in @szabo137's thesis eqs. (2.51)-(2.53).

Envisaged usage is as follows

• Define a setup S which consists of a pulse envelope (shape and width/duration)
• Define the method M used to compute the integrals (analytic, numeric)
  then computing the integrals depends on the chosen direction (of polarization/observation) pol, the background intensity a0, the photon number parameter l, and the kinematic factors alpha

phase_integral_setup =PhaseIntegral(S, M)
compute_B1x(phase_integral_setup, pol, a0, l, alpha_1x, alpha_1y, alpha_2) # other directions and integrals are named accordingly

The parameters pol, a0, ... are defined somewhere else, i.e. previously in the cross-section computation and outside QEDfields.jl.

[SimeonEhrig]

@steindev Can you please rebase. Otherwise the CI does not work.