[ENH] Tidy up unequal length collection transformers

[TonyBagnall]

fixes #1156

We have three transformers to make unequal length series equal length. One pads, one truncates and one stretches. This PR renames them Truncator, Padder and Resizer, makes the files private and reworks the associated notebook, which could ultimately be merged with the one on unequal length series.