Gate class: error handling for non-native integer types + case where control and target qubits are identical

[ValentinS4t1qbit]

Example code:

from numpy.random import randint
from qsdk.backendbuddy import Gate

# Randomly generates 2 random numbers in {0, 1}
i, j = randint(0, 2, 2)
g = Gate("CNOT",  i, j)

Currently triggers:

    if not (isinstance(target, int) and target >= 0):
        raise ValueError("Qubit index must be a positive integer.")

Suggestions:

• The error is triggered because the numpy int types are not captured by isinstance, here. To capture numpy int types, it should also include np.integer(https://stackoverflow.com/questions/37726830/how-to-determine-if-a-number-is-any-type-of-int-core-or-numpy-signed-or-not)
• Because the circuits are converted in different formats and type differences may lead to sneaky errors, I think for simplicity we should test for types (control, target, ...) and if everything is alright cast them as built-in ints in the init method.
• The error messages for target and control should be more useful and specific: Target qubit index must be a non-negative number (value: {target}) (likewise for control)
• Additional statement returning a warning if control == target. There's no reason to forbid it, but it's an odd thing to do. In this situation, the author of the code did not realize this could happen.