Add fully-functional `ParameterNode.__repr__`

[nikhilwoodruff]

Currently we read in parameters from YAML files, and calling print on a loaded ParameterNode (which often represents a single YAML file) will print out some of the original YAML read in. But a hugely impactful feature would be to have str(parameter_node) be exactly equal to the original YAML file (or better, with some sorting of keys).

This could cut runtimes in the US significantly, because currently we run lots of operations on the parameter nodes at load time (making parameter breakdowns uniform, adding missing US state values, etc.). If we could easily print out the YAML file versions of parameters we've applied these operations too, we could just commit them to the codebase and remove the need for these operations at runtime.

[nikhilwoodruff]

We should then test that this works for the root node of the UK and US systems.