Simplify core operation classes

[jlstevens]

In PR #1391 we finally started documented operations. While doing through this tutorial in #1384, we realized that we hardly ever use anything other than ElementOperation.

MapOperation has never been used to our knowledge and its definition is so light a user might as well just write their own param.ParameterizedFunction. This just leaves TreeOperation which has only been used by the holoviews devs in a few places (in particular, one operation in featuremapper is a tree operation).

My recommendation is as follows:

• Remove MapOperation entirely
• Fold the current Operation baseclass into ElementOperation
• Rename ElementOperation to Operation (a better name for users).
• Leave a class ElementOperation(Operation): definition just to issue a deprecation warning.
• Remove TreeOperation, factoring any general and useful functionality into a utility.

This plan maintains backwards compatibility as we never used or documented MapOperation and TreeOperation is very unlikely to be used by users as it is needed for rare, more complex tasks (and was also undocumented).

Having a single Operation base class would simplify things a lot!

[jlstevens]

Everything suggested in this issue has been addressed by the PR referenced above (#1421) except for removing TreeOperation.

@philippjfr Happy for me to just remove it from HoloViews and add it to featuremapper?

[philippjfr]

Sure, as long as you update the imports in topographica as well.

[jlstevens]

The final bit (removing TreeOperation) was merged in the PR above. Closing.

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