Warfarin save issues

Warfarin class has issues with its save and load functionality:

1. we have patient_counter_start which is meant to set the initial value for patient/filenames. __ini__() checks to see if save is True, but no overwrite and no use_existing are allowed, then filename_counter is set such that it does not overwrite. But this does not seem right, especially when after loading such object, we do not check these conditions.
2. _max_dose and _max_interval are determined in _determine_interval_info(). So, when loading a model with persistent interval-related features, these values are still the old ones, so the whole model is wrong!