fix "cannot bind packed field" on Ubuntu 20.04

[luator]

Description

While the old version is building on Ubuntu 18.04, newer versions of gcc
do not allow the assignment to reference here.

Note that this PR goes to mnaveau/communication_benchmark as on master this has already been fixed.

How I Tested

Build using an Ubuntu 20.04 image.

I fulfilled the following requirements

• All new code is formatted according to our style guide (for C++ run clang-format, for Python, run flake8 and fix all warnings).
• All new functions/classes are documented and existing documentation is updated according to changes.
• No commented code from testing/debugging is kept (unless there is a good reason to keep it).