Support combining neutral particles in ThOr::Combiner<T>

Once LHCb#104 is addressed and a modern/SOA-style neutral particle format is available, support will be needed in ThOr::Combiner<T>.

Here neutral particle is taken to mean a calorimeter object with no pointing information, for example a calorimeter photon, or a merged and/or resolved neutral pion. Because there is no pointing information, supporting this in the combiner should be relatively straightforward (compared to, for example, Phys#13 (moved)) in the case there are other children that can define a vertex position (example:

$D^0\rightarrow\pi^+\pi^-\pi^0$

), but it should still be tested.

There is an open question of what the combiner should do in case there is no sensible way of deriving a vertex position (example:

$\Lambda_b^0\rightarrow\Lambda^0\gamma$

).

@olupton's working assumption is that converted photons and Dalitz decays of neutral pions will be treated as composite particles with charged children.

moved from Phys#15 (moved)

added ThOr label

mentioned in merge request Moore!972 (closed)

mentioned in issue LHCb#104

mentioned in merge request Moore!1171 (merged)