Add RandomAccessibleIntervalMipmapSource4D, VolatileSource

[bogovicj]

@tpietzsch ,

@axtimwalde and I needed something like RandomAccessibleIntervalSource4D but with mipmaps. Such a source doesn't yet exist, so we made one here RandomAccessibleIntervalMipmapSource4D (in the more-4d branch.

We also add an example using the new RandomAccessibleIntervalMipmapSource4D. As well, the dev branch of n5-viewer uses it, and is tested, so I'm confident it works.

This also adds VolatileSource which is just like the existing VolatileRandomAccessibleIntervalMipmapSource, but takes any Source as an argument rather than a RandomAccessibleIntervalMipmapSource see the diff