Merge pull request #35 from mediev/master

Prandtl-Rauss plasticity model implementation
Fix support for rotated anisotropic materials in loader
Convert all tasks to use simple rheology naming ('anisotropic' or 'isotropic')
Remove strange plasticity parameters from test materials
Update materials handling in loader:
- Check if the material is used in task or not
- Check if target plasticity properties are present
- Logging takes into account both checks mentioned above
Merge remote-tracking branch 'upstream/master'
Prandtl-Rauss plasticity in stress deviator tensor terms