Missing force from nonlinear core correction

Description

Adding the force term from the nonlinear core correction. A test is added.

The following plot shows the improvement on the F2 molecule, with the following parameters (Spacing 0.25 a.u., Radius 9 a.u.).

Fixes #99 (closed)

News snippet

Adding the force term from the nonlinear core correction.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.