Add forward Euler and Tustin discretization, easily settable with the parameter _discretization_ when instantiating the _Simulation_ class. Also, make tustin default instead of backward Euler (reason: see doc of _Simulation_ class). This goes together with an overhaul of the timestep indexing - it is now far more intuitive because the _-1_ and _+1_ tricks are not required anymore. The new discretization methods come with additional tests, and documentation has been updated as well.