Remove required dependency on scikit-image and scipy

[tvercaut]

scikit-image and scipy are currently required dependencies of MONAI. However, it looks like these are used to implement a small number of features that can be implemented with PyTorch.

Limiting the number of required dependencies is good practice and for the particular case at end, replacing these implementations by PyTorch ones may help towards fixing #189

See https://github.com/kornia/kornia/blob/master/kornia/geometry/transform/imgwarp.py#L126 foe examples of replacing skimage.transform.resize and scipy.ndimage.zoom with PyTorch based code based on affine_grid and grid_sample. There is related discussion in #112

[tvercaut]

This is also linked to #290 where a related issue is highlighted but with cupy

[wyli]

this is mostly done -- scipy removed, scikit-image becomes an optional for the connected component analysis I'm closing this in favour of #554