fvOptions: Reorganized and updated to simplify use in sub-models and maintenance

fvOptions are transferred to the database on construction using
fv::options::New which returns a reference.  The same function can be
use for construction and lookup so that fvOptions are now entirely
demand-driven.

The abstract base-classes for fvOptions now reside in the finiteVolume
library simplifying compilation and linkage.  The concrete
implementations of fvOptions are still in the single monolithic
fvOptions library but in the future this will be separated into smaller
libraries based on application area which may be linked at run-time in
the same manner as functionObjects.