Generalizing terrain capability

[nataraj2]

This PR generalizes the terrain capability and is demonstrated for the King Plains AWAKEN wind farm.

1. The terrain data can be downloaded as a GeoTIFF file from USGS Earth Explorer.
2. A python script is provided that reads the data in the GeoTIFF file, converts the latitude-longitude to xy coordinates, and writes out an ERF-readable terrain file.
3. Bi-linear interpolation is used to get the z-terrain values at the mesh points.
4. Simulations with and without terrain is performed for the King Plains wind farm. The wakes for both cases are shown in the animations below. The simulation with terrain can be identified by the streaks in the inflow.
5. The power output comparison is shown as well. The min and max terrain values differ by only about 80 m for the King Plains wind farm. The resolution is 20 m. The flow is laminar as well. So, there is not much effect of terrain. Higher resolution and a realistic inflow could create significant effects.

Documentation will be added soon.