parallelization, new single_sample param, fix small bugs

[BernardaPetek]

• Parallelized image aggregation for improved performance.
• Added new parameter single_sample; when true, data_subset contains only a single sample.
• Fixed swapped output_size_px ordering (was (width, length), now correctly (length, width)).
• Removed unused plot function from base.py.
• Added DPI metadata to saved images when backend is 'matplotlib'.

[BernardaPetek]

@davidnabergoj I've unpinned all the package versions due to conflicts arising from the new jax requirement. Was there any particular reason that we had so many packages pinned to specific versions? I've unpinned everything and it seems to work fine.