Add new boundary condition MatNeumannBC

[laagesen]

Reason

Working on a problem in a solid-void system, where we would like flux to enter at specified boundary only where the solid intersects the boundary. Ideally this should be done my multiplying the desired value of flux by a switching function that is 1 in the solid phase and 0 in the void (and varies smoothly in the interface). Such switching functions already exist in the problem as material properties. Although it's currently it's possible to make a position-dependent Neumann BC using FunctionNeumannBC, it would be more convenient to create a BC that would take the material property directly.

Design

Inherit from IntegratedBC, return - _test[i][_qp] * _value * _material_prop[_qp] for ComputeQpResidual

Impact

Add something new

Tag @dschwen for feedback

[dschwen]

Seems reasonable to me.