Move forward modeling examples to own category (#72)

There will be many of these it might help to have them separated for
emphasis. Simplify the tesseroid example to only compute g_r to reduce
the run time of the example. Put on lower latitude to see the warping
effect of curvature.

examples/forward/README.txt

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+Forward Modeling
+----------------

examples/forward/tesseroid.py

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+"""
+Tesseroid Forward Modelling
+===========================
+
+Computing the gravitational fields generated by regional or global scale structures
+require to take into account the curvature of the Earth. One common approach is to use
+spherical prisms also known as tesseroids. We will compute the radial component of the
+gravitational acceleration generated by a single tesseroid on a computation grid through
+the :func:`harmonica.tesseroid_gravity` function.
+"""
+import harmonica as hm
+import verde as vd
+import matplotlib.pyplot as plt
+import cartopy.crs as ccrs
+
+
+# Get default ellipsoid (WGS84) to obtain the mean Earth radius
+ellipsoid = hm.get_ellipsoid()
+
+# Define tesseroid with top surface at the mean Earth radius, thickness of 10km
+# (bottom = top - thickness) and density of 2670kg/m^3
+tesseroid = [-70, -50, -40, -20, ellipsoid.mean_radius - 10e3, ellipsoid.mean_radius]
+density = 2670
+
+# Define computation points on a regular grid at 100km above the mean Earth radius
+coordinates = vd.grid_coordinates(
+ region=[-80, -40, -50, -10],
+ shape=(80, 80),
+ extra_coords=100e3 + ellipsoid.mean_radius,
+)
+
+# Compute the radial component of the acceleration
+gravity = hm.tesseroid_gravity(coordinates, tesseroid, density, field="g_r")
+print(gravity)
+
+# Plot the gravitational field
+fig = plt.figure(figsize=(8, 9))
+ax = plt.axes(projection=ccrs.Orthographic(central_longitude=-60))
+img = ax.pcolormesh(*coordinates[:2], gravity, transform=ccrs.PlateCarree())
+plt.colorbar(img, ax=ax, pad=0, aspect=50, orientation="horizontal", label="mGal")
+ax.coastlines()
+ax.set_title("Radial component of gravitational acceleration")
+plt.show()