Extend viscous term (with variable viscosity) to multiple quadrature rules (e.g. over-integration)

[nfehn]

Two options:

• equip ViscousKernel with multiple quadrature rules
• use a second ViscousKernel viscous_kernel_overintegration in IncNS::SpatialOperatorBase to be used in case of fully-implicit time integration with variable viscosity and over-integration for the linearized momentum operator

For this design discussion, we mainly need to take into account

• how variable coefficients are updated by a viscosity model (e.g. turbulence model), and
• how the operator is set up when used as a level-operator in MG.