Minibatched data-handling

[odunbar]

Purpose

• Closes Minibatching compatability with CES, internalize flattening operations #436

Content

• add flattening helper, and use it in get_training_points and encoder_kwargs_from. The helper takes pairs (u,g_batched) with for example with g_batch of size 3*dim_y x n_samples and converts it to ( (u,g_1), (u,g_2), (u,g_3)) where g_i is of size dim_y x n_samples
• add unit tests for the helper
• add docs to explain the behaviour with minibatching

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