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.
