Move calculating current to FDTDGrid member funcion

This calculation only uses information from the grid so makes sense to
be done by the grid class. The previous helper function was passed a
copy of the grid as a parameter already.

tests/grid/test_fdtd_grid.py

@@ -0,0 +1,121 @@
+from typing import Callable
+
+import numpy as np
+import pytest
+from numpy.testing import assert_allclose, assert_array_equal
+from pytest import param
+
+from gprMax.grid.fdtd_grid import FDTDGrid
+
+
+def get_current_in_3d_grid(
+    get_current_func: Callable[[int, int, int], float], shape: tuple[int, ...]
+) -> np.ndarray:
+    """Helper function to get current as a 3D grid
+
+    Args:
+        get_current_func: Function that returns the current value at a
+            point on a grid.
+        shape: shape of the grid
+
+    Returns:
+        result: 3D grid containing current values
+    """
+    result = np.empty(shape)
+    for i in range(shape[0]):
+        for j in range(shape[1]):
+            for k in range(shape[2]):
+                result[i, j, k] = get_current_func(i, j, k)
+    return result
+
+
+def tile(a: float, b: float, n: int = 1) -> np.ndarray:
+    """Helper function to tile two numbers
+
+    Args:
+        a: first value
+        b: second value
+        n: number of repititions
+
+    Returns:
+        c: tiled numpy array
+    """
+    return np.tile([[[a, b], [b, a]], [[b, a], [a, b]]], (n, n, n))
+
+
+@pytest.mark.parametrize(
+    "dy,dz,Hy,Hz,expected",
+    [
+        (0, 0, (1, 3), (0.5, 0.8), 0),
+        (0.1, 0.5, (0, 0), (0.5, 0.8), -0.15),
+        (0.1, 0.5, (1, 3), (0, 0), 0.2),
+        (0.1, 0, (1, 3), (0.5, 0.8), 0.2),
+        (0, 0.5, (1, 3), (0.5, 0.8), -0.15),
+        (0.1, 0.5, (1, 3), (0.5, 0.8), 0.05),
+    ],
+)
+def test_calculate_Ix(dy, dz, Hy, Hz, expected, size=2):
+    grid = FDTDGrid()
+    grid.dy = dy
+    grid.dz = dz
+    grid.Hy = tile(Hy[0], Hy[1], size)
+    grid.Hz = tile(Hz[0], Hz[1], size)
+
+    actual_current = get_current_in_3d_grid(grid.calculate_Ix, grid.Hy.shape)
+    expected_current = tile(expected, -expected, size)
+    expected_current[:, 0, :] = 0
+    expected_current[:, :, 0] = 0
+
+    assert_allclose(actual_current, expected_current)
+
+
+@pytest.mark.parametrize(
+    "dx,dz,Hx,Hz,expected",
+    [
+        (0, 0, (1, 3), (0.5, 0.8), 0),
+        (0.1, 0.5, (0, 0), (0.5, 0.8), -0.15),
+        (0.1, 0.5, (1, 3), (0, 0), 0.2),
+        (0.1, 0, (1, 3), (0.5, 0.8), 0.2),
+        (0, 0.5, (1, 3), (0.5, 0.8), -0.15),
+        (0.1, 0.5, (1, 3), (0.5, 0.8), 0.05),
+    ],
+)
+def test_calculate_Iy(dx, dz, Hx, Hz, expected, size=2):
+    grid = FDTDGrid()
+    grid.dx = dx
+    grid.dz = dz
+    grid.Hx = tile(Hx[0], Hx[1], size)
+    grid.Hz = tile(Hz[0], Hz[1], size)
+
+    actual_current = get_current_in_3d_grid(grid.calculate_Iy, grid.Hx.shape)
+    expected_current = tile(-expected, expected, size)
+    expected_current[0, :, :] = 0
+    expected_current[:, :, 0] = 0
+
+    assert_allclose(actual_current, expected_current)
+
+
+@pytest.mark.parametrize(
+    "dx,dy,Hx,Hy,expected",
+    [
+        (0, 0, (1, 3), (0.5, 0.8), 0),
+        (0.1, 0.5, (0, 0), (0.5, 0.8), -0.15),
+        (0.1, 0.5, (1, 3), (0, 0), 0.2),
+        (0.1, 0, (1, 3), (0.5, 0.8), 0.2),
+        (0, 0.5, (1, 3), (0.5, 0.8), -0.15),
+        (0.1, 0.5, (1, 3), (0.5, 0.8), 0.05),
+    ],
+)
+def test_calculate_Iz(dx, dy, Hx, Hy, expected, size=2):
+    grid = FDTDGrid()
+    grid.dx = dx
+    grid.dy = dy
+    grid.Hx = tile(Hx[0], Hx[1], size)
+    grid.Hy = tile(Hy[0], Hy[1], size)
+
+    actual_current = get_current_in_3d_grid(grid.calculate_Iz, grid.Hx.shape)
+    expected_current = tile(expected, -expected, size)
+    expected_current[0, :, :] = 0
+    expected_current[:, 0, :] = 0
+
+    assert_allclose(actual_current, expected_current)