#122 Consider having user specified properties

[Kenneth MacCallum]

I'm not sure how this would be implemented, but if properties can be added to a project, to act like variables that other object properties can depend on, then when that project is loaded as a part then those variables could be listed as properties of the part and allow adjustment of the part. Examples could be the zoom setting of a zoom lens assembly; the pupil diameter of a model of the human eye; the focus setting of a camera lens assembly, etc.

Presently all properties are accessed with an enum PropertyKey. Having user specified setting would mean possibly having to switch away from that. An alternative might be to implement an interface PropertyKey, some of which could be implemented with an enum but others could be implemented by some other means.

Needs more thought.

[Kenneth MacCallum]

This seems related to [#91]