Unify structure of QuantumObjectType

[albertomercurio]

Checklist

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Description

The current version uses both OperatorQuantumObject and Operator to handle the same type of QuantumObject. This can be a bit misleading. The same happens for all the other QuantumObjectTypes (e.g., Ket or SuperOperator).

As an example of the ambiguities given by this, in the documentation we often write something like

... where A is a Operator

but code-wise A is a OperatorQuantumObject.

Here I try to unify everything, leaving only the shorter synonyms like Operator.

We only ask the users to call QuantumObject(data, Operator(), dims) instead of QuantumObject(data, Operator, dims).