Add example of getting wind speeds at turbines (#423)

examples/18_get_wind_speed_at_turbines.py

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+# Copyright 2021 NREL
+
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+# See https://floris.readthedocs.io for documentation
+
+
+import matplotlib.pyplot as plt
+import numpy as np
+
+from floris.tools import FlorisInterface
+
+# Initialize FLORIS with the given input file via FlorisInterface.
+# For basic usage, FlorisInterface provides a simplified and expressive
+# entry point to the simulation routines.
+fi = FlorisInterface("inputs/gch.yaml")
+
+# Create a 4-turbine layouts
+fi.reinitialize( layout=( [0, 0., 500., 500.], [0., 300., 0., 300.] ) )
+
+# Calculate wake
+fi.calculate_wake()
+
+# Collect the wind speed at all the turbine points
+u_points = fi.floris.flow_field.u
+
+print('U points is 1 wd x 1 ws x 4 turbines x 3 x 3 points (turbine_grid_points=3)')
+print(u_points.shape)
+
+# Collect the average wind speeds from each turbine
+avg_vel = fi.get_turbine_average_velocities()
+
+print('Avg vel is 1 wd x 1 ws x 4 turbines')
+print(avg_vel.shape)
+
+# Show that one is equivalent to the other following averaging
+print('Avg Vel is determined by taking the cube root of mean of the cubed value across the points')
+print('Average velocity: ', avg_vel)
+print('Recomputed:       ', np.cbrt(np.mean(u_points**3, axis=(3,4))))