Task 3 is now a permeation barrier model

[RemDelaporteMathurin]

@jhdark I have completely modified task 3.

I kept the multi-material aspect of it but applied it to a real world example: permeation barriers.

Let me know what you think please

[jhdark]

This is looking great, and much more appropriate considering the feedback from the survey.

A few comments:

• I feel towards the beginning the law of chemical potential could be displayed/described
• In the materials section, perhaps the difference between sieverts and henry could be specified and in which case you would use them for people who arn't aware of them