Update 'IParticleCombiner::combine' to use 'LHCb::span<const LHCb::Particle*>'

Based on comment !4467 (comment 9802445):

(note: IParticleCombiner::combine should be updated to use LHCb::span<const LHCb::Particle*> so that one could also use eg. std::array<LHCb::Particle const*,N> here, and avoid allocating a fixed-size amount of memory on the heap)

FYI @mveghel @graven @ldufour