Summary: A general purpose formula parser & interpreter library

Description:
Ixion is a general purpose formula parser & interpreter that can calculate
multiple named targets, or "cells".

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.

Supported features:
- Each calculation session is defined in a plain text file, which is parsed and
interpreted by the Ixion parser.
- Fully threaded calculation.
- Name resolution using A1-style references.
- Support 2D cell references and named expressions.
- Support range references.
- Dependency tracking during both full calculation and partial re-calculation.
- Inline strings.
- Volatile functions. The framework for volatile functions is implemented. We
just need to implement more functions.

Maintainer: tv