Add testing of SAL core

[TOFarmer]

Topic

Tests of the SAL core submodules.

Description of Tests

Tests of:

• Symmetry between to_dict and from_dict methods, which belong to all classes which should be representable as JSON objects.
• DataClass registry
• Report classes
• path construction and validation (this already exists but definitely needs updating)
• serialisation and data encoding

Type

Unit tests.