Refactor struct name for MPD and PACMAN

[jtenavidal]

This addresses the issue opened by Roland https://github.com/DUNE-DAQ/ndreadoutlibs/issues/13

List of relevant pull requests:

• https://github.com/DUNE-DAQ/lbrulibs/pull/52
• https://github.com/DUNE-DAQ/ndreadoutlibs/pull/18
• https://github.com/DUNE-DAQ/readoutmodules/pull/59