Replace SingleValue types in data_containers.py with typing.NewType

[samtygier-stfc]

Current Behaviour

MI use subtypes of SingleValue (itself a dataclass) to provide distinct types for a range of quantities, in order to allow type checking for some function calls. These are wrappers around a base type. For example a Degree holds a float, but the type checker can prevent a Slope (also holding a float), being passed to a method that takes a Degree. There is also some code for runtime type checking in the SingleValue constructor, but this does not run for subtypes. Degrees("Hello") does not raise a error at runtime.

Desired Behaviour

Since python 3.5 the typing model has offered NewType, this has most of the static benefits with lower runtime overhead. We'd loose the ability to add constructors to do runtime checking, but we were not using this. And we loose the ability override operators, but we were not enforcing anything with this.

We would no longer need to use a.value to unpack the value, because at runtime there is no wrapping.