WIP: Rework singularity

Singularity offers "apps" these could potentially enable use like the following.

New users could get up and running with

$ singularity pull shub://lscsoft/bilby_pipe:dev
./lscsoft_-bilby_pipe-master.simg --help

I.e., ./lscsoft_-bilby_pipe-master.simg would act like the bilby_pipe executable. Then apps for "generation" and "analysis" would be used under the hood. In practise this is failing because it isn't clear how to pass the "singularity_image" file into the program.