Decomposition and computation features

Features:
- Improved update to xvec so that it can handle unitaries that decompose in more than 2 Paulis (fully integrating generic Ugates). (Includes refactoring of previous methods and functions)
- Added possibility to decompose ccx into Paulis directly (further tests needed to check if it's faster than circuit decomposition with T gates).
- Added option to calculate observable without projecting, which is costly.
- Added option to contract each added gate (costly for large depth) or leave it until the whole circuit is built (faster, but dependent on quimb's contraction heuristic)
- Added improved transformation into MPS form by extracting a minimal clifford circuit from the basis and applying it to the stabTN.
Fixes:
- Destabilizers were being wrongly referenced instead of stabilizer when applying a rotation.
- Some new attributes were not being initialized when copying the object.