Process callables and kinematics

[docbrown1955]

The libraries generated should provide callables containing the particle information (name, spin, (anti-)particle, mass, width) and default functions to load kinematics for 1->2 or 2->2 processes.

In this way we could do something like

auto const &func = f_G[i];
if (func.isDecay())
{
    // 1->2 process
   auto res = func(params);
}
else if (func.is2to2())
{
   auto res = func(params, E_CM, theta); // Automated! :)
}
else
{
   // Other type of process, fill s_ij by hand...
   params.s_12 = ...;
   // ...
   auto res = func(params);
}