Update psf model

Description

The PSF parametrization has been updated. Current formulas for scales are:

        S_{R}(r) = a - b\,r + c\,r^2 + d\,r^3\\
        S_{\phi}(r) = e\,\exp{(-f\,r)}+\frac{g}{g+r} \\

Related Issue

Closes #27 (closed)

How Has This Been Tested?

Tested manually on data and simulations

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.