Enable Papi high level stats within the iterate construct

[vidsinghal]

This is experimental and needs review.
The instrumentation is enabled via a compile time command atm.
This is the latest version of PAPI that maintained.
This would need to be installed as a dependency to use --enable-papi

[vidsinghal]

@ulysses4ever any comments on this PR?

[ulysses4ever]

Is there any chance for:

• a test showcasing the new flag; I understand numbers won't be stable across machines, but maybe there's a way to check that it's doing anything at all;

• actual documentation.

I don't know where Gibbon prefers to store docs but the --help message is not sufficient. There should be an example.

I don't feel strongly about the test because, as I mentioned, it may be tricky, but I do feel strongly about the docs.