Future features and improvements for viscoelastic material model

[naliboff]

• [x] Use general function for computing volume fractions
• [x] Replace deprecated function 'double_contract'
• [ ] Change to averaging over compositions only after viscoelastic stresses have been calculated (e.g., stresses will be averaged rather than material properties used to calculate stresses.
• [x] Add option to track viscoelastic stresses through particles
• [ ] Add a basic viscoelastic cookbook
• [ ] Determine if we can use operator splitting to enable this material model for all solver schemes
• [ ] Add a comment that the elasticity can be switched off by setting the shear modulus to a very large value
• [x] Add a benchmark for brick-experiment with viscoelastic-plastic rheology (+/- Newton functionality)?
• [ ] Implement elastic-viscous stress splitting (EVSS) method
• [x] Add a benchmark for Farrington et al. 2014 (G3) 2D shear test
• [x] Add a benchmark for Farrington et al. 2014 (G3) 3D torsion test
• [ ] Implement compressible elasticity