Unify LVH and DVH classes

[grzanka]

It seems that both DVH and LVH share a lot of code, probably the logic can be compressed into single class, see:
https://github.com/pytrip/pytripgui/tree/master/pytripgui/controller

[Yoanka]

As far as I could find, both of these classes are currently unused, in favor of using the VolHist class from base Pytrip to store data (https://github.com/pytrip/pytrip/blob/master/pytrip/volhist.py) and the Charts class from PytripGUI to display it (https://github.com/pytrip/pytripgui/blob/master/pytripgui/app_logic/charts.py).

As seen here: https://github.com/pytrip/pytripgui/blob/master/pytripgui/plan_executor/simulation_results.py#L46-L58

It appears that before refactoring, the code within these classes would have to be modified to enable using them in the Charts widget. Otherwise, it is safe to remove them.