H5MD reader does not accept box vector

[schlaicha]

Expected behavior

According to the H5MD format specification:

edges
A D-dimensional vector or a D × D matrix, depending on the geometry of the box, of Float or Integer type. If edges is a vector, it specifies the space diagonal of a cuboid-shaped box. If edges is a matrix, the box is of triclinic shape with the edge vectors given by the rows of the matrix.

The current implementation in coordinates/H5MD.py
however assumes a 3x3 matrix.
When loading a LAMMPS h5md trajectory of a cubic box, edges have shape (3,) and thus core.triclinic_box will return a vector of zeros.

Actual behavior

Box dimensions are None when loading a H5MD trajectory with box edges specified as vector.

Code to reproduce the behavior

minimal_example.zip

import MDAnalysis as mda
u = mda.Universe('minimal.data', 'lammps.h5md', convert_units=False)
print(u.dimensions)

Current version of MDAnalysis

MDAnalysis 2.4.2 / Python 3.10.6 / Ubuntu 22.04