make grid pointer part of EMfields class

[alecjohnson]

The EMfields3D class creates data that is naturally associated with a grid and therefore naturally ought to contain a reference to the grid. The grid in turn is naturally associated with a process topology and naturally ought to contain a reference to the process topology. Making these two changes allows to eliminate the ubiquitous "grid" and "vct" pointers that appear as arguments to a very large number of methods in the EMfields3D class.

Also, these classes are basically const, and therefore vct and grid should be const.

Alec