Refactor EmbeddedOdeStepper

Decouples error measurement duties from the stepper, as error
measurement is possible for non-embedded steppers too.
Add stepSize method to OdeStep
Remove redundant stepSize property

BogackiShampineStep now uses stepSize from OdeStep.
Add endState method to OdeStep

This must be in the abstract trait in order to use ErrorMeasurers in an
abstract context.
Rename a parameter

Previous parameter name reflected old type names and could therefore be
confusing.
Add errorOrder method to EmbeddedOdeStepper
Implement EmbeddedAdaptiveDtIntegrator