A-Mendi

fast_level in iec_61672_1_2013.py uses wrong filter for time weighting

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When calculating fast_level, fast/slow time weighting is to be used. For this, as the [standard](https://archive.org/details/gov.in.is.15575.1.2005) points out [Fig 1, Sec 3.5, pp. 3], the filter should have a real pole...