Document to increase CPUs

[FaroutYLq]

This is to fit the new situation on OSG, which forbids excessive CPU usage. Otherwise, typically error will be

Excessive CPU usage. Please verify that the code is configur
ed to use a limited number of cpus/threads, and matches request_cpus.

However, I didn't figure out how many threads do alea-run_toymc is trying to open. I just put an empirical 4 here for now. Still testing...

[coveralls]

Pull Request Test Coverage Report for Build 10156768279

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.167%

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Totals
Change from base Build 10066523505:	0.0%
Covered Lines:	1565
Relevant Lines:	1698

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💛 - Coveralls

[hammannr]

@FaroutYLq we can merge this as a first try. Basically this means we should test a bit with 4 and see what happens? Anything particular to look out for?

[FaroutYLq]

I already tested, and 4 CPUs generally work. (On the other hand, 1 CPU doesn't work anymore! Be alarmed). It would be better if we can find where multithreading happen in alea, but it might take a while and might be overkill. (I am not sure if OSG gonna keep the CPU rule for very long, so I think it doesn't worth the efforts now). Let's blindly do this for now: )