Materials for neutronics models

[SteSeg]

Here a materials.py file in the neutronics/ folder. I would like to:

• Add reference papers/links where they are not available
• Add tests for the ClLiF density function
• Add more materials if needed

I think @davidepettinari and I can work on this in the next days.

If we start having too many materials we can split the files in breeders.py, functional.py and structural.py or something like that. But for the moment I don't see the necessity for this.