Read laser from openPMD file

[MaxThevenet]

We currently support generating a Gaussian laser profile. Rather than implementing all complex profiles (flattened Gaussian etc.) in C++, let's have the capability to read from an openPMD file. This will be easier for advanced analytic profiles (that can then be generated in Python) and will enable arbitrary profiles, e.g. from another simulation.

At the moment, let's focus on the simple case where the input file contains the 3D profile with 1 array of complex numbers, and adopt the conventions at https://github.com/openPMD/openPMD-standard/blob/a6e997c025f4a7c564e077324c88fcc9e2dead0d/EXT_LaserEnvelope.md.

Suggested steps:

• Input a 3D file and run the simulation with the same resolution.
• The input file can have a different resolution
• Also support 2D (axisymmetric) arrays
• Same thing with azimuthal decomposition