Functions and classes not working with Qubit class

[cianibegood]

This is just to highlight the fact that currently many functions and classes can take QubitSystem as argument, but not Qubit (for instance TomoEnv). This of course has to be corrected.

[BorisVarbanov]

Hi, just took a look at this. As far as I can see this issue currently only happens in the TomoEnv (let me know if there are other instances).

A simple (and I think elegant) solution that I propose is the following: make all systems methods used by the tomography dual to QubitSystem and Qubit:

• Add qubits as an abstract property to Qubit. Then for each Qubit instance (like Transmon, Fluxonium) they'd have a simple definition, which would look like this:
  @property def qubits(self) -> List[Qubit]: return [self]
  so basically it returns a list of itself. This will allow the functions that assemble the jump operators and drives compatible. The hamiltonian syntax is already dual.

• Add the size property to Qubit class, so this is basically a direct copy-paste now.

• Finally make the functions state and state_index to be dual. These are kinda trivial for a single qubit, but I think that's ok.

So basically that should allow a single qubit to be treated as a system of... 1 qubit. Let me know what you think and I can fix this.