[BUGFIX] hardcoded reset to first turbine type in set_operation_model

[misi9170]

@Bartdoekemeijer pointed out that FlorisModel.set_power_thrust_model(), introduced in #840, seems to introduce a bug that sets all turbines to the same type.

This PR fixes that bug, and also allows users to specify different power_thrust_models for different turbines in the farm.

However, in order to do so, it must change the name of the turbine_type field on the turbine_definitions entry dictionaries. It is not immediately clear to me why this should be the case---the current operation is that if multiple entries of the turbine_definitions list all have the same turbine_type key, then only the first list entry is used.

To see this, you can comment out the lines

            turbine_type_list[tindex]["turbine_type"] = (
                turbine_type_list[tindex]["turbine_type"]+"_"+power_thrust_model[tindex]
            )

in this PR, which will cause the test to fail because both turbines will be "simple-derating".
@rafmudaf , @bayc , do you know why it's the case that the turbine_type keys need to be unique? This also came up in the example code I wrote for #851, where I had to rename the turbine_type in order for FLORIS to recognize different hub heights.

[bayc]

@misi9170 I believe we set it up that way to facilitate grabbing the turbine definitions from the turbine library. You can see here (https://github.com/NREL/floris/blob/v4/floris/core/farm.py#L113) that we build a turbine definition cache, and each turbine definition needs a unique name so that it can be mapped across all the turbines that share that type.

[bayc]

@misi9170 I believe we set it up that way to facilitate grabbing the turbine definitions from the turbine library. You can see here (https://github.com/NREL/floris/blob/v4/floris/core/farm.py#L113) that we build a turbine definition cache, and each turbine definition needs a unique name so that it can be mapped across all the turbines that share that type.

@misi9170 , ok, digging in further, it should be the case that turbines can have the same turbine_type, and shouldn't be required to be unique, so will need to dig further.