add t_statistic(), paired_t_statistic() agg functions (#15798)

[leepface]

Summary

• Added new t-statistic aggregation functions.
• Student's t distribution statistics provided by scalar functions in add Student's t distribution functions (#15798) #15809

Test plan

Tested interactively and compared results to R:

$ java -jar presto-cli/target/presto-cli-*-executable.jar --catalog tpch --schema sf10

SELECT
  t_statistic(a) AS t_a,
  t_statistic(b) AS t_b,
  paired_t_statistic(a, b) as t_paired_diff
FROM (
VALUES
  (30.02, 29.89),
  (29.99, 29.93), 
  (30.11, 29.72),
  (39.97, 29.98),
  (30.01, 30.02),
  (29.99, 29.98)
) AS t (a, b);

        t_a        |       t_b        |   t_paired_diff    
-------------------+------------------+--------------------
 19.11105607133625 | 679.298607343007 | -1.069793672096585 
(1 row)

Query 20210311_162206_00001_i4ig7, FINISHED, 1 node
Splits: 6 total, 6 done (100.00%)
0:02 [0 rows, 0B] [0 rows/s, 0B/s]


$ R 
a <- c(30.02, 29.99, 30.11, 39.97, 30.01, 29.99)
b <- c(29.89, 29.93, 29.72, 29.98, 30.02, 29.98)

# paired
t.test(b, a, paired = TRUE)$statistic
##        t 
## -1.069794

# unpaired one-sample (b) 
t.test(a)$statistic
##        t 
## 19.11106

# unpaired one-sample (b)
t.test(b)$statistic
##        t 
## 679.2986 

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== RELEASE NOTES ==

General Changes
* Added one-sample Student's t-test `t_statistic()` aggregation function
* Added paired two-sample Student's t-test `paired_t_statistic()` aggregation function

[rongrong]

Only some nits. Please also update the commit title to Add t_statistic and parired_t_statistic aggregation functions (uppercase first letter, remove ()). Thanks!

[rongrong]

the local variable is not necessary, just use updateCentralMomentsState(state, y - x); Generally speaking, if a variable is only used once, it's not necessary. Please update other places as well.

[rongrong]

You can either remove n and use state.getCount() through out, or update the other places using state.getCount to use n instead.

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. If you'd still like this PR merged, please comment on the task, make sure you've addressed reviewer comments, and rebase on the latest master. Thank you for your contributions!