Relative path to .vtu in .pvtu files

[bmxam]

This PR intends to replace absolute paths in .pvtu files by relative paths, as described in #151.

I wrote the truth table of the if block in pvtk_grid._pvtk_vtk_filename_prefix and it seems to me that what we want is simply if !relative_to_pvtk (...) else (...). At least the tests I've ran are ok.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.90%. Comparing base (74e55ff) to head (572b128).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #152   +/-   ##
=======================================
  Coverage   96.90%   96.90%           
=======================================
  Files          15       15           
  Lines         873      873           
=======================================
  Hits          846      846           
  Misses         27       27           

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[jipolanco]

Thank you!