Output attribute envelopeField to comply with laserEnvelope openPMD standard

[MaxThevenet]

According to the openPMD standard of laser envelope, the laser envelope mesh record can have any name but it must set attribute envelopeField, and its value should be normalized_vector_potential in Wake-T. It would be great to add it to the Wake-T output, as this attribute is the one used to determine whether the field read is an envelope (and, if so, which kind). In HiPACE++, this is done here.

[delaossa]

Hi Maxence,
I can take care of this in #173, whenever we finalize a first working version of the associated PR in LASY LASY-org/lasy#361