Additional validation test for ray-tracing implementation

We implemented a new test to validate our raytracing implementation.
Instead of running the N-body simulation, the snapshots at different scale factors are generated by using the same [linear power spectrum ](https://github.com/DifferentiableUniverseInitiative/flowpm/blob/9145d0c846d886f2cb34955f2267aa8cc9da31da/flowpm/tfpm.py#L159) at the scale factor a=0.5229969 (with the order set to 1).
The following image shows a comparison between the Jax-cosmo linear power spectrum (dashed line) and the [3D Power spectrum](https://github.com/LSSTDESC/DifferentiableHOS/blob/2511febab6c4c099bbc1e3937fc876bde47e2f8d/DifferentiableHOS/pk.py#L39) computed for each snapshot :

Then, we exported the lens planes, created the Convergence map and computed the angular power spectrum from it.
The final angular power spectrum is obtained averaging over 20 samples.
We also tried to investigate the behaviour of the angular power spectrum for large scale for different values of the field and Box size.

In particular we adopted the following setting:

1. Number of lens = 22 (or 11 for Boxsize =200 Mpc^3)
2. Simulated Volume = 100 Mpc^3 (or 200 Mpc^3 for Number of lens = 11 )
3. nc = 128
4. field size = 5x5 deg^2 or (10x10 deg^2)
5. Convergence map resolution = 512 (or 1024 for 10x10 deg^2)
6. Lensplane resolution: 256

The specific setting can be found [here](https://github.com/DifferentiableUniverseInitiative/flowpm/blob/0a0150629ff99c736e96182d2d50afee6c95df92/notebooks/dev/Large_scale_inv.ipynb)
To follow, the result of 4 setting combinations compared to the Jax-cosmo linear power spectrum.


The two vertical lines indicate the scale l=300 and l=3000.