[OpenSTEF 4.0] More extreme quantiles break time series plotting function

[egordm]

In evaluation, use these quantiles [0.001, 0.01, 0.05, 0.10, 0.50, 0.90, 0.95, 0.99, 0.999]. However, this throws an error when plotting the time series because line 247 in forecast_time_series_plotter.py assumes integers.

To fix. Use formatting from openstef-core instead.