use submodule for catch2

• Include Catch2 as a submodule
• Remove namespace pnm_io::, now uses only thinks::

version 0.2

• Added support for greyscale (PGM) images.
• Small update to interface.
• Added code examples.
• Tidy-up.

First release

• Modern CMake
• Tests written in Catch2
• Better adherence to Google C++ style guidelines.