indexOfMedian

Returns the index of the median in the comparable input:

• -1 if empty or all null

• index of first NaN if skipNaN is false and contains NaN

• index (lower) middle else NOTE: For primitive numbers the seq.elementAt(seq.indexOfMedian()) might be different from seq.medianOrNull()

Based on quantile implementation; uses QuantileEstimationMethod.R3.