Adding upstream version 0.19.0.upstream/0.19.0upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+Ixion is a general purpose formula parser, interpreter, formula cell dependency
+tracker and spreadsheet document model backend all in one package.
+
+## Overview
+The goal of this project is to create a library for calculating the
+results of formula expressions stored in multiple formula cells.  The cells
+can be referenced by each other, and Ixion resolves their dependencies
+automatically upon calculation.  The caller can run the calculation routine
+either in a single-threaded mode, or a multi-threaded mode. Ixion 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.
+
+## Portability
+This library is written with portability in mind; platform specific calls
+are avoided as much as possible.  It makes use of modern C++ features and the
+[boost library](http://boost.org) to achieve portability.
+
+## Performance
+Achieving good performance is one of the goals of this project.  As much
+care is taken as humanly possible, to attain reasonable performance.
+
+## Threaded calculation
+Ixion can perform threaded calculation using arbitrary number of threads,
+for both full and partial calculation modes.
+
+## 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- and R1C1-style references.
+* Support 2D cell references and named expressions.
+* Support range references.
+* Support table references.
+* 3D cell and range references.
+* Dependency tracking during both full calculation and partial re-calculation.
+* Inline strings.
+* Inline arrays.
+* Volatile functions. The framework for volatile functions is implemented. We
+  just need to implement more functions.
+* C++ API.
+* Python API.
+* Matrix support via grouped formulas.
+
+## Features known to be missing
+* More built-in functions.
+* Custom functions defined in the caller program.
+* External references.
+* Implicit intersection.
+
+## Documentation
+
+* [Official API documentation](https://ixion.readthedocs.io/en/latest/) for general users of the library.
+
+## Installation
+
+Please refer to the [CONTRIBUTING.md](CONTRIBUTING.md) file for build and
+installation instructions.
+
+
+## Download source packages
+
+Please refer to the [Releases](https://gitlab.com/ixion/ixion/-/releases) page.
+The source packages for the older versions are found [here](OLD-DOWNLOADS.md).
+
+