rewrite get_unet

[shartoo]

i have just simplified the get_unet method to make the code more elegant, and nothing others changed.

[jocicmarko]

Hi @shartoo , thank you for your PR.
Although this simplifies the code a bit indeed and makes it easier to maintain, this repository was primarily created for people who are beginners in deep learning and Keras. Thus, even though the original code is more cumbersome and not so modular as in your PR, it makes it easier to understand the architecture and correlate it with the picture of the U-Net.

Personally, I also use code similar to yours and in the long run it's much better, but for the sake of beginners' sanity I won't merge this PR. However, I will leave it open, so anyone interested in making their code more modular can take a look at your example.

Again, thank you for understanding,
Marko