Adding fit SNAP example

[emmanuellujan]

"fit_snap_example.jl":

• Uses the GaN data in the data folder
• Generate surrogate DFT data based on the GaN model presented here: https://iopscience.iop.org/article/10.1088/1361-648X/ab6cbe
• Uses snap.jl and 80% of the GaN data to create the A matrix. The matrix is generated only with the energy block.
• Uses 80% of the GaN data to create the vector b
• Uses GalacticOptim.jl to fit the parameters β, thus, solves A β = b
• Uses 20% of the GaN data and the bispectrum components provided by snap.jl to validate the fitting.
• The relative error computed during the validation is < ~2%.

ToDo:

• Check if the Gan model is correct
• Check ill-conditioned system

[emmanuellujan]

@vchuravy suggestions are welcome, thanks!