Feature proposal: particle transplantation

a function that copies particles (and potentially obstacles) across from one simulation to another makes sure all required params are there, the registrars are in a consistent state etc.
arguments would be:

• the two simulations
• a filter function (::AbstractParticle -> ::Union{Nothing, <:AbstractParticle}) to exclude/modify particles — which should be able to change the particle type as well!
• copyobstacles either a boolean or an iterable of booleans for granular control