Multisystem energies

Description

Adding energies to the multisystem framework. Each system is now computing his internal energies, and the external energy from the interactions. The classical particles are now printing their energy in a td.general/energy file.

This supports all available systems, and except for the linear_medium interaction, all the interactions.

Closes #348 (closed)

News snippet

Adding energies to the multisystem framework.

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.