Calling PhaseDiagram.as_dict() breaks the link between different Entries

[CompRhys]

Feature Requested

When saving the PhaseDiagram it could reduce the memory footprint if to keep track of the ids of entries rather than just save multiple copies of entries in the separate lists.

Proposed Solution

Refactor the PhaseDiagram class to use @property to return the different lists of entries that are kept track of based on lists of ids as opposed to making references that result in duplication when serializing.

Relevant Information

This isn't a priority and would only be worth merging if the differences in speed are negligible. Posting this issue because it may be a good first issue for someone to tackle if I can't get round to it in the near term.