Milestone: from experimental output to FBPIC/WarpX input

[MaxThevenet]

This needs the following operations:

1. Initialize a LASY profile from an experimental dataset (transverse profile at least, optionally also from measured spectrum)
2. Convert from xyt to rt representation
3. Propagate the laser if needed (maybe from near field to far field, or close enough?)
4. Write RZ output at lasy format
5. read in FBPIC/WarpX

This requires the following new capabilities (only 1 atm):

• (required, in FBPIC) Read openPMD-compliant laser file and use it in the laser antenna
• (required, in WarpX) Read openPMD-compliant laser file and use it in the laser antenna