Changing `interactionModel` through `updateOptions` does not work

[ElementW]

What was expected: g.updateOptions({ interactionModel: ... }) changes g's interaction model.
What happened: Interaction model stays unchanged.

A comment in dygraph-interaction-model.js:625-633 suggests that using g.updateOptions() to change the current interactionModel is possible:

/**
 * Default interation model for dygraphs. You can refer to specific elements of
 * this when constructing your own interaction model, e.g.:
 * g.updateOptions( {
 *   interactionModel: {
 *     mousedown: DygraphInteraction.defaultInteractionModel.mousedown
 *   }
 * } );
 */

However, dygraphs.js' structure does not allow for the dynamic change of interaction model, as it is set up in createDragInterface_(), which is only ever called once, in __init__(), and not when options are updated.

Furthermore, createDragInterface_() calls into addAndTrackEvent(), and there is no way of removing a previously added event callback aside from removing them all with removeTrackedEvents_(); thus calling createDragInterface_() again is unsuitable. That said the event it adds are wrapped (to bind this), so could be extended to act as proxies to the current, uncached interaction model.