Concatenation method

[andreped]

There are numerous scenarios where it might be of interest to apply a concatenation, especially of images/segmentations. I found that in the FlowNetwork script there is a PO called ImagesToSequence, which seems to do a concatenation along the "time" axis: https://github.com/smistad/FAST/blob/5b88fcc1d24fdf6f458f0876cd75890c25784e85/source/FAST/Algorithms/NeuralNetwork/FlowNetwork.cpp#L53

However, there is not an option to set which axis to concatenate along. It should also be easy to concatenate multiple elements simultaneously, similarly as done in numpy: output = np.concatenate([x1, x2, ...], axis=k)