Prevent that templates can be writen outside of generation directory

[yvespp]

We should check that Liima templates can only be writen to the generation directory.
Especially to prevent path traversel attacks: https://en.wikipedia.org/wiki/Directory_traversal_attack
It could be that this is already prevented.

We could prevent a leading / and .. in template paths all together or resolve a relative template path to an absolute path and then check that it is inside the generation dir.

[yvespp]

Could be implemented together with #365

[yvespp]

Closed by #595