Expand Mixture to accomodate different factor types too

[dehann]

Not just Mixture of measurement processes (as is currently already able), but different factor types too.

Anticipate NamedTuple on .mechanics field, along with required changes to serialization.

Ref:

• https://juliarobotics.org/Caesar.jl/latest/concepts/dataassociation/#Mixture-Models
• Bouncing Ball Example

[dehann]

Superseded by:

• #1420