[Feature] CPU backend for reconstruct_stack

• libertem_holo.base.convenience.reconstruct_stack() now takes a backend parameter. With backend="libertem", the reconstruction runs as a HoloReconstructUDF on a LiberTEM Context passed in as ctx, allowing for fast reconstruction on the CPU. The default remains "cupy".

• libertem_holo.base.io.InputData.into_libertem_dataset() now also supports data created from arrays, as an in-memory dataset.