UniqueString tweak

[WeiqunZhang]

Summary

Previously UniqueString used MPI_Wtime, which could have a very low resolution (1e-5s) on some machines. In this PR, we switch to use amrex::second(), which uses the highest resolution steady clock (usually 1.e-9s). We have also changed a hardwired number according to resolution of the clock so that the digits are not wasted on trailing zeros.

Additional background

ECP-WarpX/impactx#424

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate

[ax3l]

Thanks! That improves the precision we saw on macOS w/ MPI :)

[WeiqunZhang]

Somehow wget has stopped working. Maybe we should switch to using the checkout action. Will do it later.

[ax3l]

I saw some generally connection splits on CI runners today, maybe that was a temporary fluke. I see still some issues on https://www.githubstatus.com/

But yes, checkout might have higher limits due to token usage.