Consistent naming for pseudo-potential on plasma particles

[SeverinDiederichs]

This PR resolves issue #100.

Previously, the pseudo-potential at the particle position psi actually stood for psi + 1.
Now, psi actually represents psiand the 1 was added where needed (or in one case, removed).

Additionally, the field component Psi actually stood for -Psi. This was fixed, now it actually stands for Psi.

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable