Fixing datatype of value in WindData objects

[ejsimley]

Bug fix: setting value as a float in WindData objects and removing a warning when computing AVP

This PR fixes two small issues related to the WindData value array.

• First, when wind speeds were defined as integers when creating a WindData object, the values array resulting from calling the WindData assign_value_piecewise_linear method would also be integers. Since we want values to be able to take non-integer values, dtype=float is now added when assigning values.
• Second, in get_farm_AVP in floris_model.py, a warning about values being treated as 1 was printed when the WindData attribute in the floris model was a TimeSeries object. However, TimeSeries objects have a value array, so this warning isn't necessary. Now it only prints the warning when the floris model has no WindData attribute.

[paulf81]

Great catch, thanks Eric!