Add Qobj and tensor

[ytdHuang]

To make the syntax more similar to QuTiP, I suggest to add the following function support:

• Qobj(A; kwargs...) = QuantumObject(A; kwargs...) : This is just for us to create QuantumObject in a lazy way (and more similar to QuTiP). (f3c95b5)
• tensor(A...) = kron(A...) : does the same thing as kron (9949e38)
• ⊗(A, B) = kron(A, B) : take advantage of QuantumOptics (9949e38)

The last support allows us to express tensor(A, B, C) as A ⊗ B ⊗ C.

If @albertomercurio agrees with this, I can make a PR for this issue.

[albertomercurio]

Thank you for submitting this issue.

The proposed changes are reasonable. We can even try to change all QuantumObjects to Qobj, which could be implemented using a simple ctrl+f and then substituting. In this way, we will never deal with QuantumObject anymore.

[ytdHuang]

Thank you for submitting this issue.

The proposed changes are reasonable. We can even try to change all QuantumObjects to Qobj, which could be implemented using a simple ctrl+f and then substituting. In this way, we will never deal with QuantumObject anymore.

I have made a PR as shown above.

For the substitution of QuantumObject to Qobj, I didn't do this in the beginning because Julia language's naming rule prefer not to use the abbreviation.
In this PR, the new function is really defined as:

Qobj(A; kwargs...) = QuantumObject(A; kwargs...)

That means it is just passing all the args to QuantumObject() directly, and QuantumObject() will still handle all the types and other stuffs.

For me, I think defining Qobj is convenient, and QuantumObject looks nice and clear. So I like them both~

[albertomercurio]

I agree with you. Let's merge it with with both of them.