Ixa Book chapter on properties

[RobertJacobsonCDC]

Properties are sophisticated enough at this point that they probably justify their own chapter in the Ixa Book. We have also learned a few things working on ixa-epi-covid that might be worth including there.

• Use of f64 data in a property and the impl_eq_hash parameter more generally. (feat: implement Eq and Hash for Property #848)
  • When to use impl_eq_hash = both (generic generated imples) vs. impl_eq_hash = neither (manual impls) vs. OrderedFloat<f64>
• Separating data into different properties vs. storing multiple data in a single property
  • Indexing and/or querying needed? Separate fields.
  • Always accessed together? Same field gives locality, potential performance gain.
• custom display function for serialization to reports
• Types that don't fit the form expected by define_property! / define_derived_property!
• Types that are properties of multiple entities
• A warning about using type aliases with properties (Type aliases break multi-property canonicalization #843)

Unclear if in scope, but might be nice to have somewhere:

• Patterns for properties that want to be vectors (or non-Copy types more generally)
  • M-N relationships using entities and properties
  • using an index vs. using a data plugin (custom indexing strategy)
• Using a property to store the "active" value of something (latest vaccine, active itinerary, etc.)
• Using a trait extension of ContextBase / a trait on Context to define an API that hides details of property implementation.

[RobertJacobsonCDC]

Document the annoying fact that keyword arguments are ordered.