Provide interpolation function `f(x, y)` for user-driven postprocessing of results

@torrilhon pointed out that it would be nice for us to provide some sort of universal interpolation function that accepts a coordinate tuple and returns the solution state at that point, e.g., f(x, y) or f(x, y, z). This would allow users to do their own postprocessing of the results in an interactive way, i.e., create plots over line, do surface plots or contour plots etc. Based on his own experience, he suggested to use triangulation for this purpose, i.e., internally store the solution on triangles for fast interpolation.