Upgrading bind_parameters: allow in place and mixed floats + parameters

[Cryoris]

Summary

In the current situation we need to use bind_parameters to bind float values to a circuit (which returns a copy of the original) and _substitute_parameters to replace the existing parameters with new Parameter objects (which modifies the circuit in place).

With this PR the bind_parameter method gets an upgrade such that it can bind both numeric values and Parameter objects and can do so in-place or on a copy.
The signature is (type hints not actually included, just for explaining reasons here)

def bind_parameters(value_dict: Dict[Parameter: Union[Number, Parameter]], in_place: bool = False)

This is much needed functionality for circuits to become native objects in Aqua, where parameters are replaced a lot and it is cumbersome to always have to distinct between numeric values and Parameters.

Also a num_parameters attribute is added to the circuit for the same integration reason.

Details and comments

This allows to do things such as

>>> theta, x, y = Parameter('θ'), Parameter('x'), Parameter('y')
>>> qc = QuantumCircuit(1)
>>> qc.rx(theta, 0)
>>> qc.u3(0, theta, x, 0)
>>> qc.bind_parameters({theta: 2, x: y}, in_place=True)
>>> print(qc.draw())
        ┌───────┐┌───────────┐
q_0: |0>┤ Rx(2) ├┤ U3(0,2,y) ├
        └───────┘└───────────┘

[kdk]

This looks good, few comments in line. I wonder if it would make sense to rename the new method to sub_parameters or similar since binding implies a degree of finality. i.e. once bound, a circuit is ready for execution; you can only bind once; you can't pass a Parameter to execute(parameter_binds=...);...

[Cryoris]

Concerning the naming we could also use set_parameters, maybe this is the most generic. This also somewhat suggests that the behaviour is not equal to the old bind or substitute, which are still used in the parameter expression.

[kdk]

Looks good, two small comments and then this is good to go.