These additions mean that the volume-weighted average or volume integral
of a field can be conveniently post-processed. This can be done
interactively using foamPostProcess:
foamPostProcess -func "volAverage(U)"
foamPostProcess -func "volIntegrate(rho)"
Or at run-time by adding to the functions sub-section of the
controlDict:
#includeFunc volAverage(U)
#includeFunc volIntegrate(rho)