Refactor node object architecture

[TOFarmer]

Problem to solve

The architecture of how SAL describes nodes (i.e., BranchReport, LeafReport, Branch, and DataClass) has two issues:

1. While all four base classes list above can be returned from a GET request, they do not share a common interface. While they effectively implement a common interface, it would be preferable for this to be defined.

2. There is a signficant amount of code duplication, which is particularly prevalent with SummaryObject subclasses and 'DataObject' classes which contain numpy.ndarray objects.

Proposal

• Create abstract base class Node for the four different responses from nodes.
• Add abstract to_dict and from_dict to Node (from the subclasses).
• Create abstract base classes NodeReport (BranchReport, LeafReport) NodeObject (Branch, DataObject), which will unify common serialisation behaviour.

What does success look like, and how can we measure that?

• Establish a common interface for all objects that can form the response from a list or get operation.
• Remove duplication from __init__, summary, to_dict, from_dict for all DataClass subclasses.
• Remove duplication from DataObject subclasses which have array data.