Replacing the current value in a series corrupts aggregate values

[karanlyons]

> ts.create root 100
> ts.create test 100
> ts.createrule root sum 10 test
> ts.range root 0 100
(empty list or set)
> ts.range test 0 100
(empty list or set)
> ts.add root 10 1
> ts.range root 0 100
1) 1) (integer) 10
   2) "1”
> ts.range test 0 100
1) 1) (integer) 10
   2) "1"
> ts.add root 10 5
> ts.range root 0 100
1) 1) (integer) 10
   2) "5"
> ts.range test 0 100
1) 1) (integer) 10
   2) "6"

We need to store the last calculated value in aggregate contexts and then either add a replaceValue method that works off of the last calculated value as opposed to the current one, or pass timestamp to appendValue and add a branch to do what replaceValue would do.

For backwards compatibility and code style reasons I’d prefer—I think—to add replaceValue, and I’ll submit a pull doing so.