[tensorwriter] Support syst axes, up-down-axis, process-axis

[davidwalter2]

It might be good for convenience and performance wise to support input histograms with different additional axes in the add_process and add_systematic functions.

• Histograms with a process axis to set the tensor slices for different processes at once
• To support histograms with an up-down-axis for add_systematic
• To support histograms with one or more systematics-axes for add_systematic

In all these cases computations, such as symmetrizations, could be vectorized and may be more efficient.