Clarify behaviour and error messages for results.Result objects

[YHordijk]

Currently the results.Result object is not behaving ideally.

1. Accessing an empty key should return None instead of an empty Result object. Alternatively, we should raise an error
2. Accessing an empty key should not create it, only if we assign a key should we assign higher level empty keys
3. Error messages should originate from raising a KeyValue exception

[YHordijk]

I have implemented a method to get the parents of a Result object in a nice format. I then use it to give more detailed error messages when attempting to call __call__ or __hash__ of a Result object. Previously this would give a not-so-helpful "could not hash Result object" or "Result object is not callable" exception.

[YHordijk]

I have also added a prune method to the Result class. This will remove empty paths in the object, cleaning it up. This might be helpful to call before printing the object or before writing it to a file.