Add ACCUMULATOR object

[chipbarnaby]

Proposal: add a scheme that allows accumulating and summarizing the value of an arbitrary expression.

ACCUMULATOR name
acIvl = interval at which value is captured (SUBHOUR, HOUR, DAY, MONTH, (YEAR? probably not needed)
acValue = expression

Probable values:

• @accumulator[ name].<s/h/d/m/y>.whatever current interval values (*e)
• @accumulator[ name].prior.<s/h/d/m/y>.whatever prior interval values (available during next step)

.s.value OR use same items as longer intervals with all values the same

.x.sum
.x.avg (or .mean?)
.x.max
.x.min

Where .x = .h, .d, .m, .y

Issues:

• Name conflicts -- min, max ?
• Calculation timing. Must happen after other results records and meters but before reports.

[nealkruis]

I imagine for most cases, we're only going to want one type of accumulation per accumulator. Can we add a user input for accumulation type? (e.g., acType = Average/Sum/Min/Max)?

I imagine acValue will always have subhourly variability?

Can we have an input to also choose when to calculate the accumulator?

[nealkruis]

Added in #570.