Remove Singularity Container Support

[bytewife]

Hey there @lauradmartens,

I've taken over maintenance and development for the Kipoi API, models, and containers. Kipoi's singularity containers have been a hassle to store because there is a lack of a central repo for singularity containers. It would be easier to maintain Kipoi if we removed singularity support. I have a hunch that nobody or close to nobody uses the singularity API, but I'd like to get second opinions.

I think it would be wise to let Kipoi users take a survey of which API options they use (i.e. Singularity, Docker, R, Python, etc.). It can be a Google Form that we link at the top of the Kipoi pages.

Do you have any thoughts on this?