[Feature] Adding more functionality to the expression tree

[jpmoutinho]

Based on the original issue in Qadence, and respective prototype MR:
pasqal-io/qadence#399
pasqal-io/qadence#278

@dominikandreasseitz @kaosmicadei please add here what specific tree-like features should be implemented :)

[RolandMacDoland]

Some of them are already implemented but in no specific order:

• Pattern based search and replace gates in a circuit
• Parameter updates in circuits for training loops (ie. Ignacio's PR)
• Register update ? Not sure that makes a lot of sense.

[kaosmicadei]

The collect_operators is implemented using the map-reduce technique. That could be generalised to help implement other functionalities. The question is which ones and if it would be interesting to have a generic mapreduce(f_map, f_reduce, expression) implementation.