initial virtualization draft

[PeerHerholz]

This PR introduces an initial virtualization draft for snfpy. It adds the following files:

• generate_snfpy_images.sh : a small bash script that uses neurodocker to create Docker and Singularity files and also builds them locally if the user desires so
• Dockerfile: the Dockerfile created through generate_snfpy_images.sh
• Singularity: the Singularity recipe created through generate_snfpy_images.sh

Things to consider:

• test examples within image
• add a respective section in the Installation page of the docs
• push to Dockerhub, including automated builds
• tutorials on mybinder

[codecov]

Codecov Report

Merging #17 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #17   +/-   ##
=======================================
  Coverage   97.35%   97.35%           
=======================================
  Files           9        9           
  Lines         530      530           
=======================================
  Hits          516      516           
  Misses         14       14           

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