Parallel gsum and gmean

[mattdowle]

Closes #3042
First step in parallelizing gforce (gsumm.c).
gsum done. Since gmean calls gsum, mean is done too other than the extra steps for na.rm=TRUE.
No changes to tests at all, they all pass.
Code needs tidying, will come back to that.
Merging early to see how it does on server (db-bench) as I've only tested on laptop so far.

[codecov]

Codecov Report

Merging #3202 into master will increase coverage by 0.15%.
The diff coverage is 98.73%.

@@            Coverage Diff             @@
##           master    #3202      +/-   ##
==========================================
+ Coverage   94.41%   94.57%   +0.15%     
==========================================
  Files          61       61              
  Lines       11575    11739     +164     
==========================================
+ Hits        10929    11102     +173     
+ Misses        646      637       -9

Impacted Files	Coverage Δ
src/gsumm.c	90.89% <98.73%> (+3.24%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3acfd80...0a955e2. Read the comment docs.

[codecov]

Codecov Report

Merging #3202 into master will decrease coverage by 0.02%.
The diff coverage is 90.75%.

@@            Coverage Diff             @@
##           master    #3202      +/-   ##
==========================================
- Coverage   94.41%   94.39%   -0.03%     
==========================================
  Files          61       61              
  Lines       11575    11741     +166     
==========================================
+ Hits        10929    11083     +154     
- Misses        646      658      +12

Impacted Files	Coverage Δ
src/gsumm.c	88.59% <90.75%> (+0.94%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b4d4783...670cce3. Read the comment docs.

[jangorecki]

https://h2oai.github.io/db-benchmark/
well done