Add cell info into the header in XYZWriter

[ChiahsinChu]

Is your feature request related to a problem?

I found that the XYZ files written by MDAnalysis do not include the cell info in the header, even if the dimensions are not None. It would be inconvenient if I wanted to use the XYZWriter for format conversion with cell parameters varying throughout the trajectory.

Describe the solution you'd like

I'd like to include the cell information into the header of the XYZ file if the dimensions are None so that this information can be read again in another package (e.g., ASE). I can check the dimension info and modify the remark in the XYZWriter.

Additional context

Introduction of ExtXYZ format: https://wiki.fysik.dtu.dk/ase/ase/io/formatoptions.html#extxyz