Add a NaN init option for temp fields
Temp fields are now recycled to optimize initialization of fields in GPU devices. The draw back is that we cannot rely on default zero init anymore.
In order to help detect problem linked to that initialization relaxation, it might be usefull to add a solution to optionally init these fields to NaN. That option could be used systematically in regression testing.
@elena this is related to #680 (closed) and #667