Add PMT_NVML_LEGACY_MODE option
Determine at CMake time whether to enable the NVML legacy mode or not, instead of at compile-time.
This also allows the user to pass -DPMT_NVML_LEGACY_MODE=On
at the command-line.
This solves an issue encountered when using the NVML backend on LEONARDO, where the current method with Field Values doesn't result in sensible energy measurements.