Add nuclear gradient functions

[msricher]

This PR adds nuclear gradients for the overlap integrals, the core Hamiltonian ($\hat{T} + \hat{V}_{ne}$) integrals, and the electron repulsion integrals, in the AO basis.

I based this on pyscf pyscf/grad/rhf.py, although that code is for an optimized Hartree-Fock wave function (in the MO basis). Once I am able to test these, the code will be ready to merge.

Checklist

• Write a good description of what the PR does.
• Add tests for each unit of code added (e.g. function, class)
• Update documentation
• Squash commits that can be grouped together
• Rebase onto master

Type of Changes

	Type
✓	✨ New feature

[PaulWAyers]

I would start by adding a test for Cartesian Gaussian primitives. When you take the derivative of a Gaussian primitive with respect to its nuclear coordinate, you pull down a multiplicative factor of $-\alpha | \mathbf{r} - \mathbf{R}_A|$, so each component of the gradient has an extra factor of the corresponding Cartesian dimension. That's an easy test.

The contracted Gaussian functions are similar (just a sum over primitive contributions.

The final bit is the AO -> MO transformation which can be tested vs PYSCF I think.