Add stop_when_flux_decayed termination criteria

[oskooi]

Adds a new stop_when_flux_decayed termination criteria which is similar to the existing stop_when_dft_decayed. Also includes a unit test.

The new termination criteria is particularly useful in cases involving glancing-angle waves which are poorly absorbed by PML (and for which the existing termination criteria stop_when_fields_decayed and stop_when_dft_decayed tend to require long run times which can cause field instabilities).

As an example, a planewave incident at oblique angles on a binary grating produces high-angle diffraction orders which tend to be slowly decaying within the periodic unit cell. Tracking the convergence of the Poynting flux in the direction of the PML absorption (i.e., $z$ direction for a 3D unit cell periodic in $x$ and $y$) enables terminating the simulation within a reasonable time (although choosing the tol parameter based on empirical testing requires some care).