The goal of this project is to create a library for calculating the
results of formula expressions stored in multiple named targets, or
"cells".  The cells can be referenced from each other, and the library
takes care of resolving their dependencies automatically upon
calculation. The caller can run the calculation routine either in a
single-threaded mode, or a multi-threaded mode.  The library also
supports re-calculations where the contents of one or more cells have
been modified since the last calculation, and a partial calculation of
only the affected cells need to be calculated.
