[TI.by_speed] function showing TI plot without clarifying where IEC characteristics are coming from

[BiancaMorandi]

Is your feature request related to a problem? Please describe.

The TI.by_speed function is generating a TI by wind speed plot with IEC characteristics lines but it is not clear what is the IEC reference. see plot below.

TI_plot, TI_data = bw.TI.by_speed(data_clnd[wspd_main], data_clnd[wspd_main + '_sd'],
                                  percentile=90, return_data=True)

Describe the solution you'd like
I suggest that

1. function used for plotting is enhanced to report on legend the IEC reference used for these characteristics. According to code my understanding is that these IEC characteristics are defined based on IEC 61400-1-2005 parameters.
2. The docstring description is enhanced to better describe where are the IEC parameters used for the plot coming from

[BiancaMorandi]

@stephenholleran note that I have added this issue as for enhancement requested by a client.