[Idea] JIT VariableType (aka Phantom Variables)

[dehann]

It should be possible to infer the minimum necessary VariableType at the very last moment, and could result in user only using addFactor as observations/data happen. This will make better use of factor timestamps rather than have the whole system centered around variables and variable timestamps -- i.e. variables become driven aspects, where factors are the prime aspects.

xref:

• AddFactor should not require the variables to exist #835
• Change to VariableDFG.missionnanosec alongside .timestamp #1087