A DOM-like / object-oriented API for N5

[bogovicj]

Discussed with @axtimwalde:

N5's API uses Strings (relative URIs) to refer to groups and datasets, but
we could consider adding an N5 api that reflects the fact that an N5 hierarchy is a DOM.

• N5Node
  • getPath returns an N5URI
  • getParent returns an N5Node (or null if this is the root)
  • getAttributes
• N5GroupNode
  • getChildren returns a N5Node[]
    • this could go in N5Node if it can return (null if it is a leaf)
• N5DatasetNode
  • getDimensions
  • getBlockSize
  • getDataType
  • getCodecs
  • (any other Dataset-specific metadata)

In this case N5DatsetGroup would essentially be DatasetAttributes with additional information about the hierarchy.
Similarly, the N5DatasetNode could have N5Reader (and N5Writer?) methods for reading (and writing?) blocks and shards.

@cmhulbert