add test coverage and more docs for DRT

[dehann]

TODOs

• Improve test coverage in IIF, https://codecov.io/gh/JuliaRobotics/IncrementalInference.jl/src/master/src/TetherUtils.jl
• Spruce up related docs in Caesar, https://juliarobotics.org/Caesar.jl/latest/examples/deadreckontether/

RoME already has Pose2 test

https://github.com/JuliaRobotics/RoME.jl/blob/master/test/testDeadReckoningTether.jl

Notes

important to establish convention of:

fg = initfg()

addVariable!(fg, :x0, Pose2, tags=[:POSE;], solvable=0) # or Pose3
# ... add factors etc

# set that graph fragment solvable=1

addVariable!(fg, :drt_0, Pose2, tags=[:POSE; :DRT], solvable=0)
drec0 = MutablePose2Pose2Gaussian(...) # or InertialPose3
addFactor!(fg, [:x0; :drt_0], drec0, solvable=0, graphinit=false)

# now find high frequency solution with:
accumulateDiscreteLocalFrame!
accumulateFactorMeans

# when its time to trigger a new pose, just drop a new variable with convenience function
duplicateToStandardFactorVariable

EDIT, there is an example video too: https://juliarobotics.org/Caesar.jl/latest/examples/examples/#Towards-Real-Time-Underwater-Acoustic-Navigation