Separate config and data storage in FactorGraph object design

[dehann]

Currently FactorGraph contains both.

[dehann]

cc @GearsAD

[dehann]

Hi @GearsAD, just bumping this thread since we had discussed it at some point - good place to capture the thoughts if you have any. Currently we are defining the Variable data elements in a wiki as follows:
https://github.com/JuliaRobotics/Caesar.jl/wiki/Data-Design-for-Caesar-RoME-IIF

[dehann]

xref #133