Xdmf generation (h52xdmf.py) on Windows does not work

[sanathkeshav]

On Windows, the absolute path to the h5 file in the XDMF file causes problems.

also see: nschloe/meshio#302

[sanathkeshav]

Solution suggested in the GitHub issue says to put the h5 file and the xdmf file together and remove the path from the xdmf file.. this solution works but is not a general solution.

Found by @s5409693

[strinner213]

More general solution (if it is not wanted/possible that h5 and xdmf file are in the same directory): Completely avoid absolute paths within the xdmf-file and only use relative paths to h5 files. Seemed to solve the Windows issue for me in an unrelated test at least.