create class Mask about pathml

[jacob-rosenthal]

__getitem__(self, item) should take integer and __len__(self) should return number of masks, so that you can loop through masks easily (e.g. for mask in slide.masks: do something)

But it would also be nice to be able to access masks by their names, e.g. slide.masks["tissue_detection"]

I think a good approach might be:

• keep the masks stored in internal dict, like mask._mask_dict
• __len__(self) = len(self._mask_dict)
• getitem takes either integer or string input
  • if integer, return self._mask_dict.values()[item]
  • if string, return self._mask_dict[item]

[ryanccarelli]

opened branch @dev-masks