fixed grad reg for SCAN functionals, calculate p directly

[JFurness1]

Description

Fixed regularisation of SCAN-like functionals for small gradients.

Motivation and Context

This solves the issue of SCAN-like functionals returning NaN when the gradient is small. See issue #144.

How Has This Been Tested?

Updated code passes previously set tests.

Screenshots (if appropriate):

Todos

N/A

Types of changes

• [x] 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)

Questions

Status

• [x] Ready to go