Fixes for the species jellium slab and tutorial

Description

Fixes for the species jellium slab:

• The electrostatic energy was wrong
• The slab can be now shifted by the position of the species

The formula are also documented and rewritten in hopefully a cleaner way.

A test is added

Closes #22 (closed) Closes #926 (closed)

News snippet

Improvements and fixes for the jellium slab species.

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.