Add VTK Matrix/Basis Generators, Multilinear Forms, and High-Order Material Matrices

[davschneller]

• refactor the given mass matrix, stiffness matrix, etc. to using a general multilinear form function which allows for more general and powerful function evaluation
• add VTK point generators, and collocation matrices for them
• add high-order material matrix generators (which basically just utilize the new multilinear form generator in a slightly extended way)