Move checksum library to a separate repo?

[MaxThevenet]

WarpX and HiPACE++ currently have their own copy of our checksum library: https://github.com/Hi-PACE/hipace/tree/development/tests/checksum. They diverged slightly, but nothing hard to fix. We briefly discussed putting this on a separate repo, to make sure contributions to it are beneficial to both codes easily. Also, it would make sense if other codes were interested in using it. I open this issue to start a discussion, and have a few questions to help with the decision:

1. Do we want to support multiple output format, or just openPMD? We currently support plotfile and openPMD.
2. (somehow related to 1) Would other codes consider using it? e.g. AMReX @WeiqunZhang @atmyers, FBPIC @RemiLehe or Wake-T @AngelFP?