DynamicMap callback return types should be validated

[philippjfr]

When working with DynamicMaps and returning Overlays it is possible that the keys in the Overlay can change, which makes it necessary to reassign the Element to a matching plot instance. Reassigning plots in this way will work when PR #367 is merged but since no global ordering can be computed during initialization of the plot it's not possible to assign the correct cyclic index in this case. We should detect this condition in the plotting code and warn about it, which should then alert the user by displaying the warning in the browser (https://github.com/ioam/holoviews/issues/372).

Additionally we should validate the type and number of returned Elements in an Overlay and warn the user if one or more types could not be rendered because they were not declared during initialization.

[philippjfr]

This issue is no longer relevant since the underlying restriction has been addressed.

[github-actions]

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