Fix Chirp

[k-wolfinger]

Logic has been broken, needs improvement

[k-wolfinger]

Need to ask the user for

• tau_0: the Fourier-limited pulse length of a given spectral bandwidth
• tau_c: the chirped pulse length

Then, the chirp parameter, b, can be calculated as b^2 = (tau_c / tau_0)^2 - 1

[k-wolfinger]

Typical application includes a positive chirp (red, low frequencies leading blue, high frequencies) followed by amplification and then a negative chirp (inverted frequency ordering to compensate for the positive chirp).

[k-wolfinger]

This can be achieved by the following equation: omega_chirp = omega_0 + (b*t/tau_c**2)

Where omega relates to the photon energy through the wavelength lambda: omega = 2 pi c / lambda

In our case, t=0, and z varies from positive for the front laser slice to negative for the last slice.

[k-wolfinger]

Implemented a linear chirp, explored in the rslaser_scratch notebook https://github.com/radiasoft/rslaser_scratch/blob/main/notebooks/propagate/chirp_check.ipynb