ExprEval group_by and array_reduce

[Daniel Besse]

Example:

; from array durations={days: duration}
month_groupby = @(d) date("mY", d);
month_duration = group_by_keys(@month_groupby, durations);
; month duration contains {"012016": [0 12 24 4], "022016": ...}
month_duration = array_reduce(@sum, month_duration)
; month_duration contains {"012016": 40, "022016": ...}

[Daniel Besse]

ExprEval time_range, array_groupby, array_reduce (fix #120)

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