Add method to return turbine TIs

[bayc]

Feature or improvement description
This PR adds a method to return the calculated turbulence intensities in the flow field on the turbine rotors.

Related issue, if one exists
Closes #442

Impacted areas of the software
floris_interface.py

Additional supporting information
This will return TIs at the flow field points on the turbine rotors. This will be the averaged TI across the points on the turbine rotor.

Test results, if applicable
The following code prints out the returned TI values at each of the 3 turbines for the 2 wind conditions specified.

Code:

from floris.tools import FlorisInterface

fi = FlorisInterface("inputs/gch.yaml")
fi.reinitialize(wind_directions=[270., 0.], wind_speeds=[8.0])
fi.calculate_wake()

print(fi.get_turbine_TIs())

Output:

[[[[[0.06020304]]

   [[0.09741578]]

   [[0.11112154]]]]



 [[[[0.06020304]]

   [[0.06021817]]

   [[0.06022236]]]]]