Add N-center overlap integrals

[aamrindersingh]

Adds support for computing overlap integrals of arbitrarily many Gaussian basis functions.

The main addition is n_center_overlap_integral(basis, n, ...) which generalizes the existing 2-center overlap to arbitrary N. It uses Obara-Saika recursion with memoization for the angular momentum build-up, and includes screening based on the K factor from the Gaussian product theorem.

Usage:

from gbasis.integrals.overlap_n_center import n_center_overlap_integral

S3 = n_center_overlap_integral(basis, n=3)
S4 = n_center_overlap_integral(basis, n=4, tol_screen=1e-10)

I've added a regression test to ensure that the N=2 case matches the existing overlap_integral() exactly. All 11 new tests pass and existing tests are unaffected.

The implementation follows the patterns in overlap.py and should serve as the foundation for intracule/extracule functionality mentioned in the issue.

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
	🐛 Bug fix
✓	✨ New feature
	🔨 Refactoring
	📜 Docs

Related

Closes #220