Implementation of a hit multiplicity trigger

The MR implements a trigger on hit multiplicity. The procedure is identical to the digi multiplicity trigger, only with a hit vector as input instead of a digi vector. The generic algorithm TimeClusterTrigger is used on the time stamps extracted from the hit data.

In the current implementation of algo::evbuild::Config, the Hit Multiplicity trigger is used when the node v0trigger is not found and the node hit_mult_trigger is found.