Additional documentation for simulation in cylindrical coordinates

[oskooi]

This PR expands the existing documentation of the cylindrical-coordinates feature in Exploiting Symmetry/Cylindrical Symmetry to include:

• listing of Maxwell's equations in cylindrical coordinates
• schematic of the Yee grid
• discussion of the special boundary conditions at $r = 0$
• description of the conversion factor necessary to convert the Poynting flux computed in cylindrical coordinates to 3d Cartesian coordinates (in case this may be useful for certain types of calculations)

[stevengj]

Good to merge?

[oskooi]

I still need to update the figure of the Yee grid to swap the $r$ and $z$ axes following @smartalecH 's suggestion.