Make TTN feature complete

[PabloAndresCQ]

Implement the abstract functions from StructuredState not yet supported by TTN. These are measure, sample, postselect and add_qubit.

[PabloAndresCQ]

A first rough implementation for postselect appears in branch feature/ttn_postselection and add_qubit appears in research/compilation (along with some stuff not relevant to this issue). Some cleaning up would be needed, along with implementation of the remaining methods.