Add calculation of the Scatter Index statistic

[TatianaBurek]

Describe the New Feature

dtcenter/MET#1843 adds 3 new columns to the CNT line type (SI, SI_BCL, and SI_BCU), for Scatter Index with bootstrap lower and upper limits. This new output is in MET version 10.1.0 and will be included in the MET version 10.1.0 beta2 development release.

The equation for the Scatter Index is very easy: SI = RMSE / OBAR

Like many other CNT statistics, SI is derivable from the SL1L2 line type. It should be handled in the same way that other CNT statistics, like RMSE, are handled.

Related METdatadb issue:
dtcenter/METdataio#61

Related METviewer issue:
dtcenter/METviewer#317

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