[ESSSANS] Add code for computing PolyGauss instead of using a file

[SimonHeybrock]

Currently we have PolyGauss_I0-50_Rg-60.h5 stored in our data. According to Judith this is simple to implement in code (avoiding the need to track and download a file), e.g., using SciPy. Judith will provide code for this.