Improvements for load_duration_curve-method

[eivindno]

Load duration curves in the Power-sdk needs some improvements.
If query a load_duration curve where there are more than one timeseries that satisfies the arguments in the method, I receive an error message in returns.
For instance.
sogndal_ledning = client.ac_line_segments.search(name = 'Sogndal')[5]
sogndal_varighet = sogndal_ledning.load_duration_curve(start = '52w-ago', dropna = False, granularity= '10m', measurement_type='PhaseVoltage', timeseries_type='value', terminal = 2)

I would like to have all three excising time-series returned as load-duration-curve data-frame. With the name of the Time-series in the column-name (Also where there is only one exist time-series.).
So where there is more than one time-series for timeseries_type and measurement_type please return all of them in a data-frame.

I would also like to be able to determine the aggregation-method. With interpolation as default.

Hope you are able to fix this.
Thank you!

Eivind