[BUG] Debian package has outdated libpng dependency

[Matthew]

The Debian package that James has kindly provided on Bridgewater-Brunel specifies a dependency on an older version of libpng. This version of the library is not available in recent versions of Ubuntu, and I imagine other Debian distributions too.

Steps to reproduce:
1. Run Ubuntu 16.04 or more recent.
2. Download the latest Debian package.
3. Attempt to install using dpkg.

The latest version of the required package is libpng16-16 and I was able to install and run Extended on Ubuntu 18.04 by changing the "depends" line to "libpng16-16" (and making other changes to be reported in separate bug reports).