cleaning
*added function to create an Hamiltonian operator
* default stepsize in exponential operator
* added function that resurns a time propagator
* wrapper around the exponential operator to define exponential time
evolution operartor
'LR' = 'RL'
etrs propagator
Op.expr = Opin.expr was preventin operator coposition to work properly
in the etrs propagator
with etrs we can use bigger time-step