add log_softmax operation

[sfluegel05]

Describe your changes

I needed a translation of the LogSoftmax operation into keras, so I added it here.

Checklist before requesting a review

• [x] I have performed a self-review of my own code
• [x] I have verified that the code builds perfectly fine on my local system
• [x] I have added tests that prove my fix is effective or that my feature works
• [ ] I have commented my code, particularly in hard-to-understand areas
• [x] I have verified that my code follows the style already available in the repository
• [ ] I have made corresponding changes to the documentation