Feature/improvementsIncrementalFilter

Created a pull request on this branch from @lucacarlone et al, mainly to see differences as well as ping Luca on resolving this

• starting to add remove functionality in fixedLagSmoothers, test does not pass..

• fixed unit test!

• added remove functionality in BatchIncrementalSmoother with test

• targets for unit tests

• added print function to FixedLagSmoother::Result struct

• added method to access isam2 results in incremental fixed lag smoother

• added unit test for determinant in Bayes tree

• added get to access gravity vector

• Add jacobians to retract functions of Unit3 and OrientedPlane3

• Specify key involved when throwing cheirality exception