Single particle QED emission

[haykh]

Structural changes

• srpic.hpp is now subdivided into different dedicated header files with explicit functions.
• Pusher_kernel now accepts an additional template argument: class E = NoEmissionPolicy_t<SimEngine::SRPIC, M>.

Major fixes/enhancements

• Synchrotron and Compton emission implemented.
• FIXED boundaries for fields now properly transform the passed field components from tetrad basis to code units.

Minor fixes/enhancements

• support for 2D arrays in input file
• -D TESTS=ON can now be used together with pgen to also compile all unit tests.
• GRPIC now compiles when no init_flds is present in pgen

Emission

Emission is performed by passing a class to the pusher with the following two mandatory methods:

Inline auto EmissionPolicy::shouldEmit(
                 const coord_t<M::PrtlDim>&,
                 const coord_t<M::PrtlDim>&,
                 const vec_t<Dim::_3D>&,
                 const vec_t<Dim::_3D>&,
                 const vec_t<Dim::_3D>&,
                 vec_t<Dim::_3D>&,
                 Payload&) const -> Kokkos::pair<bool, bool>;

which returns two booleans to indicate whether a particle should be emitted and whether the emitting particle should experience a recoil (i.e. radiative drag); and

Inline void EmissionPolicy::emit(
                 const tuple_t<int, M::Dim>&,
                 const tuple_t<prtldx_t, M::Dim>&,
                 const vec_t<Dim::_3D>&,
                 real_t,
                 real_t,
                 const Payload&) const;

which actually does the emission. Here the type EmissionPolicy::Payload is defined within the class, and is used to store a temporary variable (such as the energy of the potentially emitted photon) and pass it between the functions (separate per each emitting particle).