Renamed:
  fieldFunctions.* <-- multilayer.*
  numericalIntegration.cpp <-- fluxIntegrals.cpp

Also a major cleanup of (what is now called) fieldFunctions.cpp.  There
is a simpler interface for the fluxes.  Importantly overloaded the
primary flux() function so that SMatrix can be precomputed once and
reused.  Lower-level functions gfFlux() allow TE/TM specificity, and
also incorporate flux rates from single emitter location, rather than
integrated over whole layer.

Fluxes still don't work if s==l (i.e. source in flux layer).

Next: add a density-of-states function to fieldFunctions.cpp