Add methods to access volume data for algorithms like marching cubes

[kephale]

Description

This PR addresses issue #607 by adding utilities for accessing the original volume data and current timepoint view.

Implemented:

1. A VolumeUtils class with static methods:

   • getOriginalRandomAccessibleInterval(volume): Retrieves the original data
   • getCurrentView(volume): Gets a view of the current timepoint

2. Extension methods for the Volume class:

   • volume.getOriginalRandomAccessibleInterval<T>()
   • volume.getCurrentView<T>()

3. Example command VolumeMarchingCubes showing how to use these methods to apply marching cubes at the current timepoint

4. Documentation explaining usage

Benefits

• Provides a clean API for accessing the original data without relying on "magic keys" in metadata
• Returns properly typed data for the current timepoint, simplifying usage with algorithms like marching cubes
• Shows a complete example of how to use these utilities in a command
• Works with both time series and regular volumes

Testing

I've tested this with both regular volumes and time series volumes, ensuring that:

• The original data is correctly retrieved
• The current timepoint view is correctly created
• The marching cubes algorithm works properly on the current timepoint

Fixes #607