This workspace makes use of the internal TraceTree of each cavity in the simulation
trace forest to re-compute round-trip ABCD matrices (and dependencies, such as
eigenmode, stability) if necessary. The ABCDSolution objects previously stored in
Cavity are gone now as this solution is much cleaner and faster - also means caching
of cavities in run_axes_scan of runners.pyx is no longer required.

Duplication of code between Cavity and CavityWorkspace remains for now. This will be
changed so that both use a single C based version (currently CavityWorkspace implements
a fast C version of each update function whilst Cavity implements some Python methods
for updating eigenmodes / stabilities and such).

Also added in this commit is a work in progress trim method for TraceForest which
will remove the need for checking traversed nodes during tree propagation beam tracing.