Xdmfdiff timesupport

With MR 3246! an xdmf may contain data for multiple time steps. With this version we can now choose for which time step the data is compared. The default timestep is 0.

Example cmd line:

xdmfdiff -a v -b v --timestep-a 0 --timestep-b 0 ~/comparevtuxdmf/cube_1e3.xdmf ~/comparevtuxdmf/cube_1e3_2.xdmf

or with default timestep

xdmfdiff -a v -b v ~/comparevtuxdmf/cube_1e3.xdmf ~/comparevtuxdmf/cube_1e3_2.xdmf

Further this MR contains 2 bug fixes. (see commit messages).

Tests will be added in separate MR.