Simultaneous solvers use same CSMC / Tree?

[dehann]

Picking up from https://github.com/JuliaRobotics/IncrementalInference.jl/issues/459#issue-529964851

[dehann]

Note, building a tree requires temporary changes in the factor graph object and hence only one tree can be built at a time, however, multiple trees can be used simultaneously on the same factor graph object after they have been built.

For most flexibility it might be best to try support both models as far possible (short of rebuilding the tree building process in the short term).

[dehann]

For trees, work towards a DFG style where TreeClique has all data common all solvers, but then TreeCliq contains a field as ::Dict{Symbol, BTND} used for each of the different solvers. This implies elements like potentials (#680) should move up to TreeClique, etc.