Nudge people to the default chunk_size setting

[ctk21]

This PR nudges people to use the default chunk_size for parallel_for. In all the cases I've seen the default chunk is superior in our sandmark numerical benchmarks, particularly when the number of domains gets large. This is often because tasks can turn out to be more imbalanced than people expect: a domain does GC marking, is slower due to cache effects, has a hiccup getting scheduled by the OS, etc.

I have left the parallel initialization examples as is. I don't have experience to know if experiments have determined a small chunk_size as best for these.

[Sudha247]

Thanks @ctk21! Definitely makes sense to move to the default chunk_size. Will run some experiments on parallel intialization examples, we can probably change them to default chunk_size as well if the results are comparable.

[Sudha247]

Results on IITM machines show comparable results for default chunk size vs hard coded value for the parallel initialization examples.

DLS version (float_init_par3)

Cores	Hard-coded	Default
1	3.09	3.07
2	1.56	1.56
4	0.81	0.88
8	0.43	0.43
12	0.3	0.3
16	0.25	0.24
20	0.22	0.2
24	0.19	0.2

Array of Random States (float_init_par2)

Cores	Hard-coded	Default
1	6.16	6.15
2	5.78	8.18
4	4.55	2.19
8	2.21	1.17
12	1.28	0.75
16	2.21	0.71
20	1.49	0.57
24	1.46	0.59

It might be a good idea to stick to default chunk size on the initialization examples. We can address that later on #7, needs a rewrite after the standard library random module changes are merged. Merging this one now.