Functional 2d + 3d code

Major changes:

• Add 3d example notebooks for two datasets
• Correct min_object_size (evaluated now from looking at the train and validation masks)
• Save tif images with datatype np.uint16 (in the prediction notebooks )
• Provide support in case evaluation GT images are not available (during prediction)

Some things which are still incorrect in v0.2.0:

• n_y should be set to n_x for equal pixel/voxel sizes in y and x dimension. This is fixed in v0.2.1
• anisotropy_factor is wrongly calculated for the 3d notebooks (it was calculated as the reciprocal). This is fixed in v0.2.1
• train_size was set to 600 for the bbbc012-2010 dataset. This is raised to 1200 in v0.2.1