Specify im-seg mask greyscole mode
Author: mishig25
Pull request: yes

# What does this PR do?

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Fixes # (issue)
After testing the output of image-segmentation pipeline as part of image-segmentation widget tests, found out the need to specify to pillow that `image-segmentation` pipeline outputs greyscale images (i.e. single channel images, black & white)

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Comments:
@Narsil merging it now.
However, I plan to do some research on support of boolean images by various libraries.
As the first use case, I would like to test whether boolean images are supported in HTML (for the widget use case). 
I would like to know whether I can do something like this:
```js
const img = document.getElemendById('some-image-element-id');
img.src = 'data:image/png;base64,  BASE64 str of boolean image'
```