Minor improvements in multigrid time summary

[lpsaavedra]

Description

This PR adds the workload_imbalance(...) and the vertical_communication_efficiency(..) calls to output important information for the comparison of the local smoothing and global coarsening approaches.

In addition, minor problems related to MG output were found:

1. The timers for mg detailed times were created by specifying the MPI Communicator, however, this does not output the correct min, max and average times.
2. The initial condition time was somehow overlapping with other timers.
3. The update_multiphysics_time_average_solution() was being called when the meshes were refined, however, this is only needed if more than one physics is active.

Solution

1. The MG timers are now constructed without the MPI communicator.
2. An independent timer was created for the ramp initial condition and it prints the wall time at the end of the ramp.
3. An if condition checks whether other physics are active.

The last two were solved for both the matrix-based and matrix-free application.

Testing

This was tested locally by using the benchmarks for locally refined meshes.

Documentation

Nothing needs to be modified.

Checklist (will be removed when merged)

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Code related list:

• The branch is rebased onto master
• Changelog (CHANGELOG.md) is up to date
• Code is indented with indent-all and .prm files (examples and tests) with prm-indent

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• The PR description is cleaned and ready for merge