ExpRampWithPrepulse Temporal Envelope for PIConGPU in new interface

This example shows how one can implement the old ExpRampWithPrepulse Temporal Envelope Profile for GaussianPulse for the new Free Functor interface using sympy. The interface change was introduced in #5437.

Steps:

1. Setup parameters in generate_laser_profile.py and run the script.
2. Verify the profile by looking at the plot in laserTemporalProfile.png.
3. Copy the laserTemporalProfileExpresion.txt into the functor in incidentField.param. For reference have a look at the FreeEnvelopeFunctor in the attached example param file.

Notice

1. Once the free functor is added to picmi/pypicongpu it will be possible to directly pass the sympy expression to picmi.
2. The way ExpRampWithPrepulse was implemented is not ideal.If the relative ramp intensity is is to high the preceding part of the main gaussian pulse will disappear in the ramp.