Relative path to .vtu in .pvtu files

[bmxam]

Hi,

First of all, thank you for this great package that we use every day.

I am aware of #139 and #141, but my problem is slightly different. I would suggest an option to write relative paths to .vtu files in the .ptvu file. Today, when we write a .pvtu file by calling pvtk_grid with an absolute path, the created .pvtu file contains absolute paths to each .vtu file: pvtk_grid("/foo/bar/result", ...) creates a file /foo/bar/result.pvtu containing

<?xml version="1.0" encoding="utf-8"?>
<VTKFile type="PUnstructuredGrid" version="1.0" byte_order="LittleEndian">
  <PUnstructuredGrid GhostLevel="0">
    <Piece Source="/foo/bar/result/result_1.vtu"/>
    <Piece Source="/foo/bar/result/result_2.vtu"/>
    <PPoints>
      <PDataArray type="Float64" Name="Points" NumberOfComponents="3"/>
    </PPoints>
  </PUnstructuredGrid>
</VTKFile>

Then the problem is that the whole folder containing the results cannot be moved since the paths in the .pvtu are absolute. I would rather expect a .pvtu containing relative paths (to the .pvtu itself, not the current working dir of course):

<?xml version="1.0" encoding="utf-8"?>
<VTKFile type="PUnstructuredGrid" version="1.0" byte_order="LittleEndian">
  <PUnstructuredGrid GhostLevel="0">
    <Piece Source="result/result_1.vtu"/>
    <Piece Source="result/result_2.vtu"/>
    <PPoints>
      <PDataArray type="Float64" Name="Points" NumberOfComponents="3"/>
    </PPoints>
  </PUnstructuredGrid>
</VTKFile>

In other words, I think the .pvtu file should be constructed like the .pvd file : the path indicated by the user indicates where to create the file, but the paths to the .vtu or .pvtu inside the .pvd are relative to the .pvd filepath.

I am ok to propose a PR if you agree with this remark.