Optimize HDF5 I/O performance by using collective I/O operations

[sanathkeshav]

As the title suggests, we use parallel HDF5 I/O to write field data. Previously, it was serialized and was in general very suboptimal and slow when it came to large problems. This PR provides a fix along with a bit of decluttering the postprocessing function in the solver.

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.