PNGwriter: Add Spack Package

Adds a package for
[PNGwriter](https://github.com/pngwriter/pngwriter/),
a simple high-level C++ png API used in scientific projects.

```
PNGwriter is a very easy to use open source graphics library that
uses PNG as its output format. The interface has been designed to be
as simple and intuitive as possible. It supports plotting and reading
pixels in the RGB (red, green, blue), HSV (hue, saturation,
value/brightness) and CMYK (cyan, magenta, yellow, black) colour
spaces, basic shapes, scaling, bilinear interpolation, full TrueType
antialiased and rotated text support, bezier curves, opening existing
PNG images and more.
```

PNGwriter is a dependency for [PIConGPU](http://picongpu.hzdr.de),
an open-source, many-core, fully-relativistic particle-in-cell code
and further software developed at
[Helmholz-Zentrum Dresden - Rossendorf](https://www.hzdr.de).