Added cleaner callback mechanism, removed obsolete code
Cleaned-up version of vortex street ipython notebook
Simulation code (non ipython notebook)
Compute momentum map (preliminary code -- bug somewhere)
version of momentum map on S3
fixed bugs in calling convention for momentum map on S3
(temporary) debugging constructs
Finished implementation of momentum map

Switched order of one-step integration methods

Fixed bug in evaluation of gradient vector fields at projected midpoint
Streamlined definition of Hamiltonian on S2 and S3, as well as their gradients.
Now everything is defined in terms of "inner products", i.e. H_S2 is propotional
to the log of 1 - x_i x_j, and everything is derived from there.