feat: implement magnetic dipole field calculation

[majockbim]

Summary

This PR implements the foundational physics layer for magnetic field simulation by adding the dipole field calculation function. This is a prerequisite for grid iteration and field line tracing.

Changes

See (#2)

Physics Implementation

The field $\mathbf{B}$ is calculated as:
$$\mathbf{B}(\mathbf{r}) = \frac{\mu_0}{4\pi} \cdot \frac{1}{r^3} [3(\mathbf{m} \cdot \hat{\mathbf{r}})\hat{\mathbf{r}} - \mathbf{m}]$$
where $r = r_{obs} - r_{dipole}$.

Verification Results

Automated Tests (GTest)

• Point Singularity: Returns zero vector when $r_{obs} = r_{dipole}$.
• On-Axis Accuracy: Field direction aligns with the dipole axis for points along the axis of $\mathbf{m}$.
• Inverse-Cube Law: Verified that $|\mathbf{B}|$ scales with $\frac 1{r^3}$ across varying distances.
• Superposition: Confirmed that the total field from two dipoles equals the vector sum of their individual fields.

Constraints

• Observation points within 1e-9 units of a dipole are treated as being at the dipole center to avoid numerical instability.