changed .vtk to .vtu to prevent read error in CI due to file version

tests/agard.py

@@ -48,7 +48,7 @@ def main():
 
     # I/O
     matreadr = ardr.AsciiReader(os.path.join(os.path.dirname(__file__), 'data', os.path.basename(__file__)[:-3], 'structural')) # matrices
-    modreadr = vrdr.VtkReader(os.path.join(os.path.dirname(__file__), 'data', os.path.basename(__file__)[:-3], 'structural'), 'vtk') # modes
+    modreadr = vrdr.VtkReader(os.path.join(os.path.dirname(__file__), 'data', os.path.basename(__file__)[:-3], 'structural'), 'vtu') # modes
     prsreadr = vrdr.VtkReader(os.path.join(os.path.dirname(__file__), 'data', os.path.basename(__file__)[:-3], 'aero'), 'vtu') # pressure
     writer = awrt.AsciiWriter()