add back correct(!) warning about LHCbIDsToMCParticles not being threadsafe

Let's make sure that anyone using LHCbIDsToMCParticles is aware of the fact that it suffers from data races, and thus will not be guaranteed to give correct results when run multithreaded.

Basically, any use of this tool should be replaced by an algorithm that prepares the required information, and a (trivial) tool that accesses that information (at which point the tool is probably overkill)