CarpetX's openPMD output code includes lohman/json.hpp which is not installed by openPMD_api

[Roland Haas]

This leads to compile time failures:

COMPILING CarpetX/CarpetX/src/io_openpmd.cxx
/home/jenkins/workspace/CarpetX/configs/sim/build/CarpetX/io_openpmd.cxx:18:10: fatal error: nlohmann/json.hpp: No such file or directory
   18 | #include <nlohmann/json.hpp>

removing the offending requires that one hand-constructs the correct json options string which should be easily possible

[Roland Haas]

A hard-coded version is in here

[Roland Haas]

Applied as git hash 8fb0f27 "CarpetX: hard-code json option string for openPMD" of cactusamrex