Merge pull request #3 from at0m741/codex/ajouter-readme.md-pour-bssn

Add README for BSSN headers thank to OpenAI Codex

Author: at0m741

includes/Tensorium/DiffGeometry/BSSN/README.md

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+# Tensorium — BSSN Module
+
+This directory contains the building blocks for the BSSN (Baumgarte–Shapiro–Shibata–Nakamura) formulation used in numerical relativity. The headers implement helper routines to compute conformal variables, derivatives and curvature terms from a given spacetime metric.
+
+## Header Overview
+
+- `BSSNSetup.hpp` – defines `BSSNGrid` and the `BSSN` class. It initializes all BSSN variables at a spatial point: lapse, shift, conformal metric \(\tilde{\gamma}_{ij}\), its derivatives and inverse, Christoffel symbols, the extrinsic curvature \(K_{ij}\) and the trace–free tensor \(\tilde{A}_{ij}\).
+- `BSSNMetricUtils.hpp` – utilities for generating conformal metric fields on a 3D grid (e.g. `generate_conformal_metric_field`).
+- `BSSNDerivatives.hpp` – finite difference helpers to compute partial derivatives of scalars, vectors and tensors.
+- `BSSNChristoffel.hpp` – computes the conformal Christoffel symbols \(\tilde{\Gamma}^{k}_{ij}\) and provides routines to differentiate a 5‑D metric field.
+- `BSSNTildeChristoffel.hpp` – contracts the conformal Christoffel symbols with the inverse metric to form \(\tilde{\Gamma}^{i}\).
+- `BSSNContractedChristoffel.hpp` – evaluates the contracted connection \(\Gamma^{i}_{ij} = -\tfrac{3}{2}\,\partial_j \ln\chi\).
+- `BSSNextrinTensor.hpp` – computes the extrinsic curvature tensor \(K_{ij}\) from metric time derivatives, lapse and shift.
+- `BSSNAtildeTensor.hpp` – builds the trace‑free conformal extrinsic curvature tensor \(\tilde{A}_{ij}\).
+
+## Status
+
+The BSSN implementation is experimental and currently limited to initializing variables at a single grid point. Evolution equations and advanced gauge conditions are not yet available, and derivative operators rely on straightforward finite differences. APIs may change as development continues.
+
+## Usage
+
+`setup_BSSN_grid` from `includes/Tensorium/Functionnal/FunctionnalRG.hpp` provides a simple entry point:
+
+```cpp
+using tensorium::Vector;
+Vector<double> X(3);       // (x, y, z)
+X(0) = 10.0; X(1) = 0.0; X(2) = 0.0;
+
+tensorium_RG::Metric<double> metric("kerr", 1.0, 0.8);
+double dx = 0.1, dy = 0.1, dz = 0.1;
+
+auto bssn = tensorium::setup_BSSN_grid(X, metric, dx, dy, dz);
+
+const auto& gamma_tilde = bssn.grid.gamma_tilde[0];
+const auto& A_tilde     = bssn.grid.A_tildeTensor[0];
+```
+
+The returned `tensorium_RG::BSSN` object stores all conformal and curvature quantities for that spatial coordinate.