Implement composite lexicographic less operator for TPs, TAs, TCs

[alessandrothea]

At the moment less is based on simple timestamp comparison.
The effect is that TPs/TAs/TCs are not captured correctly by readout requests.
Appropriate less operators need to be implemented for TPWrapper, TAWrapper and TCWrapper classes in the respective T<X>Buffers declarations.

[ArturSztuc]

Not closing yet, but this is already completed for TPs and TAs, where the less operator is based on both the time, and the channel (channel_start in case of TA).

For TC this is based on the time_start and type. This is not ideal if we have two TCs at the same time, of the same type.
TCs do not require to have TAs, and therefore any locality information (e.g. timing events), so this might need more thinking before implementing.

I'm currently thinking of something along:

   bool operator<(const TCWrapper& other) const
      {
        if (this->candidate.inputs.size() && other->candidate.inputs.size()) {
          return std::tie(this->candidate.time_start, this->candidate.type, this->candidate.inputs.begin()->channel_start)
                  < std::tie(other.candidate.time_start, other.candidate.type, other.candidate.inputs.begin()->channel_start);
        }
        return std::tie(this->candidate.time_start, this->candidate.type) < std::tie(other.candidate.time_start, other.candidate.type);
      }

but it seems messy.