h5::array_stack writes a sequences of arrays of the same shape (or of scalars) into an hdf5 array with one more dimension, unlimited in the stacking direction.
It is typically used to store a Monte-Carlo data series for later analysis.
* If the base of the stack is an array of rank R, the resulting hdf5 array will be of rank R+1.
* If the base of the stack is a simple number (double, int, ...), R=0.