Test dynUNet for 2D fetal brain segmentation

[martaranzini]

Test MONAI implementation of dynUNet on our fetal brain segmentation task to check its performance compared to the nnU-Net framework.
Follow the tutorial: https://github.com/Project-MONAI/tutorials/blob/master/modules/dynunet_tutorial.ipynb

Note: need to upgrade to monai0.3.0 to use the dynUNet.

[martaranzini]

WIP in branch 5-dynUNet