Add tutorial notebook: FCHK to electron density on a grid (IOData, Grid, GBasis)

[ModaHarsh]

Summary

Adds a small tutorial notebook that demonstrates an end-to-end workflow using IOData, Grid, and GBasis: loading a wavefunction from a Gaussian .fchk file, building a molecular grid, and evaluating/integrating the electron density.

What the notebook does

• Section 1 (IOData): Loads data/h2o_sto3g.fchk, prints geometry and basis info.
• Section 2 (Grid): Builds a Becke-style molecular grid.
• Section 3 (GBasis): Builds the 1-RDM from the wavefunction, evaluates the density on the grid, and integrates to get the electron count (~10 for H₂O).

Notes

• Uses the existing notebooks/data/h2o_sto3g.fchk example.
• Includes an optional first cell for Colab (install + download data).
• Working-directory handling so the notebook runs from repo root or from notebooks/.
• Validates mol.mo before use when building the 1-RDM (aligned with horton3.ipynb).

This was proposed as a first contribution following maintainer guidance on adding tutorial notebooks to Horton3.

[ModaHarsh]

Hello @PaulWAyers,

I hope you’re doing well! This PR adds a tutorial notebook demonstrating an end-to-end workflow with IOData, Grid, and GBasis. I believe it aligns with the project’s goals and would be helpful for users.

When you have a moment, I’d appreciate your review or feedback. Let me know if there’s anything I can adjust.
Thankyou!