Simplify convertDomPointToCursorStep method

This ```convertDomPointToCursorStep``` method on ```OdtDocument``` has a parameter that lets the caller provide a function to customise the rounding behaviour. In practice the callers of this method simply use the default fallback of rounding down or provide the same duplicate function to get round up behaviour.

This PR simplifies the interface by only providing an option to either round down or round up when converting to steps. This does not touch the ```OdtStepsTranslator``` interface so any class that might need custom rounding behaviour in the future can still access it directly from the steps translator.
Okay. Please also put a "Default: core.StepDirection.NEXT" directly to the explanation of the parameter `roundDirection`.