Additional example to study metal grating convergence

[mfschubert]

It would be useful to have additional examples which demonstrate the convergence characteristics of the various FMM formulations supported by fmmax. In particular, the crossed metallic grating discussed in #34 could be a good choice. That grating comes from example 3 of Li's paper.

This task could largely be achieved by copying and existing example (e.g. metal grating) and modifying it so that the correct geometry is modeled. The example should also reproduce the figure from Li's paper.