Summary
Clarify the interface for serialising `Serialisable` instances. Serialising now clearly means converting instances to a string, while a new `to_primitive` method converts instances to a dictionary of python primitives.
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Contents
Refactoring
- [x] Only allow `Serialisable.serialise` to serialise to strings
- [x] Factor new `Serialisable.to_primitive` method out from `Serialisable.serialise` and use it to replace previous usages of `Serialisable.serialise(to_string=False)`
Operations
- [x] Allow `test` branch prefix and remove the old `release` prefix
Testing
- [x] Test datafiles with spaces in name work as expected
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