UNet 3+ model added

[hamidriasat]

Two changes

• New model UNet 3+ added
• Reference also added in README.md

[divamgupta]

Hi thanks for the PR, but it seems like that the new model you added might not work with other functionality of the repo. For e.g. it is not added in the all_models.py , hence cli tools wont be able to use it.

[hamidriasat]

Hi @divamgupta, thanks for your reply.
New changes have been made in code to make it compatible with rest of the repo functionality. Please test it with cli tools. I hope it works this time, if anything is missing please let me know.

Following changes have been made in the current commit:

• Unet3+ model reference added in all_models.py
• Code formatting in unet3_plus.py to make sure local methods are not accessible to outside classes/objects
• PEP8 code formatting of test_models.py to make sure it's easily readable
• Unet3+ reference added in test_models method of test_models.py file