Add `cum_count()`, `cum_avg()` , and `cum_std()` to `update_by()`

[alexpeters1208]

Given that we offer exponential weighted average and std, and rolling count, avg, and std for update_by(), I think it's reasonable to support cum_count(), cum_avg(), and cum_std(). These are natural things to want to compute, and I'm having a tough time computing them by composing existing operations.

Here is the gap in cumulative/rolling operations compared to Numerics.java array operations:

• count (cumulative, rolling already exists)
• avg (cumulative, rolling already exists)
• count_neg, count_pos, count_zero (can implement using modification to CumSpec)
• abs_avg
• std (cumulative, rolling already exists)
• wstd
• var
• wvar
• cov
• cor
• ste
• wste
• tstat
• wtstat
• median
• percentile

[lbooker42]

A review of Numerics.ftl/Numerics.java might uncover other functionality gaps.

[chipkent]

Other count operations like count_null, count_nan, etc. would be useful.