Adding functions to set wind direction by upstream turbines

[ejsimley]

Feature or improvement description
This pull request addresses issue #235 by adding functions set_wd_by_upstream_turbines and set_wd_by_upstream_turbines_in_radius to the dataframe_manipulations module. The reason for these functions is that wind directions measured by waked turbines can contain biases due to wake effects. Therefore, it could be advantageous to only use unwaked turbines to estimate the wind direction. These functions first use the average wind direction over all turbines to identify the unwaked turbines. Then the unwaked turbines (or unwaked turbines within the specified radius for set_wd_by_upstream_turbines_in_radius ) are used to compute the final wind direction signal.

Related issue, if one exists
This PR closes issue #235

Impacted areas of the software
The dataframe_manipulations module

Test results, if applicable
Tests added in "dataframe_manipulations_test.py"

[paulf81]

This looks great @ejsimley , nice and clean!