add ability to retrieve ROI names in a case insensitive way 

HI,
i'm experiencing problem with low quality ROI labels (written like this: Heart, heart, HEART), 
maybe an option to extract case insensitive ROIs could be useful to other also

I have a quick_fix in rtstruct.py:98
```if structure_roi.ROIName.casefold() == name.casefold():```

maybe a more general option while using ".get_roi__mask_by_name" could be better
