[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.

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[TonyBagnall]

Looks good, but needs deprecation. Pretty sure everything can stay as it is now just add the old classes and __init__ with a warning

I thought that collections was experimental? Conflicted with deprecation in any case, might just leave this until after the next release

[MatthewMiddlehurst]

Series is but collection is not. Have it listed in the README