Multisystem propagation log

Description

The iteration information printed to the log is now handled at the level of system_t, as all systems should by default print the same information during a time-propagation (iteration, energy, etc). It basically reproduces the log in the old code.

Also cleaned a bit the log, as there were several places where the namespace was not used, plus some beautification.

News snippet

Improved multisystem propagation log.

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.

Closes #353 (closed)