Moved reltol and abstol to optional parameters in ode()

While using the simplified ode() function interface I ran into problems because the precision was not enough  for running my simulation. I changed the code so that the tolerances can now be specified as optional parameters of the function. This doesn't break any code and I think it's common/useful enough to be of used.

I've also included a link to a ijulia notebook that is using this code. I think this would be useful to other users.