Fix the meshesPath for openPMD 1.1

According to the openPMD standard 1.1, the openPMD file should contain the metadata meshesPath (which indicates where the fields are stored) if (and only if) the file contains field data.

When implementing openPMD 1.1 (see https://bitbucket.org/berkeleylab/warp/pull-requests/164/conform-to-the-new-110-openpmd-standard/diff), I forgot to include this in the BoostedFieldDiagnostics, and this causes the openPMD-viewer to fail, when reading the corresponding file. The current PR writes the metadata meshesPath in the function create_empty_meshes, which is used both in FieldDiagnostics and BoostedFieldDiagnostics.