Add reference to an image in forward notebook (#16)

Add a missing reference to Alagbo et al. (2022) for weed management in
ridge tillage systems (Figure 5). See: Alagbo, O.; Spaeth, M.; Saile,
M.; Schumacher, M.; Gerhards, R. 'Weed Management in Ridge Tillage
Systems—A Review.' Agronomy 2022, 12, 910.
https://doi.org/10.3390/agronomy12040910.

notebooks/01-forward-dc-resisitivity-2d.ipynb

@@ -76,7 +76,7 @@
    "source": [
     "## Step 1: Defining Topography\n",
     "\n",
-    "Defining the topography is a good first step when setting up the forward simulation. It gives the user a sense of the scale of the problem and is easy to plot. For a 2.5D simulation, topography is defined as an (N, 2) *numpy.ndarray*, where the first coordinate represent along-line position, the second coordinate represents the vertical position, and we assume the topography does not change along the a 3rd orthogonal direction. The 2.5 simulations works well in modeling cases like the one pictured below.\n",
+    "Defining the topography is a good first step when setting up the forward simulation. It gives the user a sense of the scale of the problem and is easy to plot. For a 2.5D simulation, topography is defined as an (N, 2) *numpy.ndarray*, where the first coordinate represent along-line position, the second coordinate represents the vertical position, and we assume the topography does not change along the a 3rd orthogonal direction. The 2.5D simulations work well in modeling cases like the one pictured below (Alagbo et al., 2022, Figure 5), which is provided under the CC-BY-4.0 license.\n",
     "\n",
     "<img src=\"https://www.researchgate.net/profile/Oyebanji-Alagbo-2/publication/359861220/figure/fig1/AS:1143365911478274@1649611084348/Schematic-description-of-ridge-tillage-with-crops-placed-on-top-of-the-re-compacted.png\" width=\"70%\" align=\"center\"/>\n",
     "\n",