Update private functions and docstrings in savu.core.utils

It should be easier for you to do this. It's up to you how much detail you want to go into, but it might be useful for functions used by developers to document like this:
""" Get all padded slice lists.

```
:param Data data_list: datasets
:param list(list(slice)) slice_list: slice lists for datasets
:param int count: frame number.
:param dict squeeze_dict: squeeze functions for datasets
:returns: all data for this frame and associated padded slice lists
:rtype: list(np.ndarray), list(tuple(slice))
"""
```

which is the format I've been using