Update for slice_viz function

The slice_viz function has been updated with the following new features:

• By default position is None, and slice_viz plots an overview of the entire stack.

• If position is given as a string or integer, slice_viz will plot an overview with n_slices figures around that position.

• If position is given as a list or array, n_slices will be ignored and the idxs from position will be plotted.

• The default img_height and img_width have also been increased, and the title in the plots now scale wrt these inputs too.

In this merge request is also included:

• Change to test_slice_viz (default heights have changed)
• Updates for several docstrings from previous implementations.