Added simple VQE sample

[DmitryVasilevsky]

This PR adds simple self-contained example of a Variational Quantum Eigensolver (VQE).

This example includes:

1. Simple classical optimization to find minimum of a multi-variable function in order to find the approximation to the minimum eigenvalue of a Hamiltonian.
2. Finding Hamiltonian expectation value as a weighted sum of terms.
3. Finding one term expectation value by performing multiple shots of preparing ansatz state and measuring it.
4. Ansatz state preparation similar to the circuit in the published paper "Ground-state energy estimation of the water molecule on a trapped ion quantum computer" (https://arxiv.org/abs/1902.10171)

To keep this sample simple and self-contained Hamiltonian is artificial (doesn't correspond to any actual chemistry problem). Also, this sample doesn't include any application of VQE, for example, finding a geometry of a molecule.

[msoeken]

Looks good to me, left some comments.

[swernli]

Looks good; I think it would be worth adding it to the samples.mjs that controls which samples show up in the editor!

[DmitryVasilevsky]

... adding it to the samples.mjs that controls which samples show up in the editor!

Not sure if this is a good sample to have as a quick template. Not all samples are in that list, and this sample is a more specialized one.

[swernli]

... adding it to the samples.mjs that controls which samples show up in the editor!

Not sure if this is a good sample to have as a quick template. Not all samples are in that list, and this sample is a more specialized one.

Ok, we can leave part for a later PR if we decide we want to include it.