[BUGFIX] Warn users if the turbine_type is set without setting reference_wind_height

[misi9170]

Replaces #809 to address #806. When a FlorisModel is instantiated using an input wind file that has a reference_wind_height: -1, the hub height of the specified turbine model is used as the reference wind height. However, if the turbine_type is subsequently changed, the reference wind height is not updated to match. This PR adds a warning that is raised if turbine_type is set without also setting the reference_wind_height.

The following script demonstrates this update. Prior to the fix, the following code runs without raising any warnings:

from floris import FlorisModel
#from floris import UncertainFlorisModel as FlorisModel
#from floris import ParFlorisModel as FlorisModel

# Initialization uses NREL 5MW model with 90m hub height
fmodel = FlorisModel("../inputs/gch.yaml")
print(fmodel.core.flow_field.reference_wind_height)

# Changing the turbine type without specifying reference_wind_height
fmodel.set(turbine_type=["iea_15mw"]*len(fmodel.layout_x))
print(fmodel.core.flow_field.reference_wind_height)

# Changing the turbine type and specifying reference_wind_height
fmodel.set(turbine_type=["iea_15mw"]*len(fmodel.layout_x), reference_wind_height=150.2)
print(fmodel.core.flow_field.reference_wind_height)

producing

90.0
90.0
150.2

After the bugfix, the output is instead:

90.0
floris.floris_model.FlorisModel WARNING turbine_type has been changed without specifying a new reference_wind_height. reference_wind_height remains 90.00 m. Consider calling `FlorisModel.assign_hub_height_to_ref_height` to update the reference wind height to the turbine hub height.
90.0
150.2

TODO:

• Implement warning
• Check that warning is also raised for UncertainFlorisModel and ParFlorisModel (other warnings are also raised due to reinstantiation of the underlying fmodel_expanded with UncertainFlorisModel, but this is acceptable)
• Add test for raising warning