Dockerfile support

Created by: JCGoran

Allows gevolution to be built and run with Docker, which can be useful for just running the code without the need to install any additional packages or libraries on the machine (except Docker itself). Of course, it can also be used for standalone code development.

The current version is based on Debian Stable, and builds gevolution with CLASS and HEALPix support, as well as the lccat and lcmap utilities.

For build instructions and example usage, see this README file. There's currently a pre-built image available on Dockerhub in case users don't want to waste time building it from the Dockerfile, or are using an environment which doesn't have Docker, but has alternatives such as shifter or Singularity available.

I've tested it on various machines, using both Docker and Singularity, and haven't found any major issues so far (except for gevolution occasionally complaining about a missing output directory, which is easily fixable).