feat(4d): replace timepoint with dimensionGroupNumber for 4D handling with backward compatibility

[sedghi]

In our 4D handling, we previously used timePoint (starting at 0) and numberOfTimePoints. However:

1. It is confusing because it starts at 0 instead of 1.
2. It is incompatible with the fact that the 4th dimension can represent something other than time (e.g., positionIdentifier). Here is the relevant part of standard

This PR introduces dimensionGroupNumber in a backward-compatible way to prevent breaking changes.

If you see a warning in your console, follow the instructions to update accordingly. In Cornerstone3D 3.0, all references to timePointIndex will be removed in the future.

Note on 0-based and 1-based Indexing:

• timePoints were 0-based
• dimensionGroup is 1-based

Old Name	New Name	Context
timePointIndex (property/setter in IDynamicImageVolume, StreamingDynamicImageVolume)	dimensionGroupNumber (property/setter in IDynamicImageVolume, StreamingDynamicImageVolume)	Interface, Class
numTimePoints (property in IDynamicImageVolume, StreamingDynamicImageVolume, ImageVolume, VoxelManager.createScalarDynamicVolumeVoxelManager)	numDimensionGroups (property in IDynamicImageVolume, StreamingDynamicImageVolume)	Interface, Class, Static Method
getCurrentTimePointImageIds	getCurrentDimensionGroupImageIds	Class (StreamingDynamicImageVolume)
flatImageIdIndexToTimePointIndex	flatImageIdIndexToDimensionGroupNumber	Class (StreamingDynamicImageVolume)
isTimePointLoaded	isDimensionGroupLoaded	Class (StreamingDynamicImageVolume)
markTimePointAsLoaded	markDimensionGroupAsLoaded	Class (StreamingDynamicImageVolume)
DYNAMIC_VOLUME_TIME_POINT_INDEX_CHANGED	DYNAMIC_VOLUME_DIMENSION_GROUP_CHANGED	Enum (EVENTS)
DYNAMIC_VOLUME_TIME_POINT_LOADED	DYNAMIC_VOLUME_DIMENSION_GROUP_LOADED	Enum (EVENTS)
checkTimePointCompletion	checkDimensionGroupCompletion	Class (StreamingDynamicImageVolume, BaseStreamingImageVolume)
timePoint (parameter in VoxelManager.createScalarDynamicVolumeVoxelManager)	dimensionGroupNumber (parameter in VoxelManager.createScalarDynamicVolumeVoxelManager)	Static Method
setTimePoint	setDimensionGroupNumber	Class (VoxelManager)
getAtIndexAndTimePoint	getAtIndexAndDimensionGroup	Class (VoxelManager)
getTimePointScalarData	getDimensionGroupScalarData	Class (VoxelManager)
getCurrentTimePointScalarData	getCurrentDimensionGroupScalarData	Class (VoxelManager)
getCurrentTimePoint	getCurrentDimensionGroupNumber	Class (VoxelManager)

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