Merge #496

496: Create OHLC aggregate and accompanying accessors r=rtwalker a=rtwalker

Begins to address #445. 

Introduces OHLC aggregate called as `toolkit_experimental.ohlc(timestamp, price)`. 

The following accessors are provided for use with the `OpenHighLowClose` aggregate:
* `open`, `high`, `low`, `close`

along with the timestamp-returning versions:
* `open_at`, `high_at`, `low_at`, `close_at`

Additionally, there is a new `toolkit_experimental.rollup()` aggregate that takes `OpenHighLowClose` aggregates as input.

In the event that multiple rows contain the highest or lowest value, only the earliest occurrence is retained. Similarly, should multiple rows contain the opening or closing timestamp, this implementation only updates the open/close points with a new value when the new timestamp is strictly earlier/later than current timestamp.

Additional work on #445 remains to address handling volume (and possibly creating technical indicators).

Co-authored-by: Ryan Walker <rwalker@timescale.com>

extension/src/lib.rs

@@ -19,6 +19,7 @@ pub mod time_vector;
 pub mod frequency;
 pub mod saturation;
 pub mod countminsketch;
+pub mod ohlc;
 
 mod palloc;
 mod aggregate_utils;