Custom particle BCs

[LudwigBoess]

This PR adds the option to define custom particle BCs in the problem generator

[LudwigBoess]

As an example, this is how you would implement a reflecting BC that resamples the particle velocity on reflection in the pgen:

    struct CustomPrtlBC {
      random_number_pool_t pool;
      real_t temp_cold, temp_hot;

      template <class PusherKernel>
      Inline void operator()(index_t p, int dim, bool is_min, const PusherKernel& pusher) const {
        vec_t<Dim::_3D> v {ZERO};

        // Reflecting boundary that resamples velocity
        if (dim == 1) {
          if (is_min) {
            arch::JuttnerSinge(v, temp_cold, pool);

            pusher.i1(p)  = 0;
            pusher.dx1(p) = ONE - pusher.dx1(p);
            pusher.ux1(p) = v[0];
            pusher.ux2(p) = v[1];
            pusher.ux3(p) = v[2];
          } else {
            arch::JuttnerSinge(v, temp_hot, pool);

            pusher.i1(p)  = pusher.ni1 - 1;
            pusher.dx1(p) = ONE - pusher.dx1(p);
            pusher.ux1(p) = v[0];
            pusher.ux2(p) = v[1];
            pusher.ux3(p) = v[2];
          }
          
        }
      }
    };

    template <class D>
    auto CustomParticleBoundary(simtime_t /*time*/, spidx_t sp, D& domain) const {
      return CustomPrtlBC{
        domain.random_pool(),
        temperature / domain.species[sp].mass(), 
        temperature_gradient * temperature / domain.species[sp].mass()
      };
    }