Make step group optional within a file

[f-schmitt]

Allow user to change the internal file structure to avoid the timestep group. Required to use splash files with VisIt. Add an internal flag to state that this group is missing to allow transparent reading of such files.

[ax3l]

@psychocoderHPC did not open the xdmf file, but the plain hdf5 file - nevertheless we should check again if everything works out with that one.

[f-schmitt]

@psychocoderHPC is this still an issue?

[psychocoderHPC]

Not tested with visit and xdmf. I hope I can test it tomorrow.