Make package structure more hierarchical

[bogovicj]

We should consider making the package hierarchy less flat, something like:

• org.saalfeldlab.janelia.n5
  • compression
  • datablock
  • keyvalueaccess

moving relevant classes into those packages. The bases n5 package could have classes like the N5Readers and Writers, DatasetAttributes, and not much else.

ping @tpietzsch , @cmhulbert

[bogovicj]

In my opinion, this feature can wait until after the 4.0.0 milestone