Core refactoring
Fix #10 (closed) by using a unique Core object, which will use the specific Runner to run computation (locally, or remotely).
If user set a computation server it uses the RunnerSsh, else if user set a Slurm server it uses the RunnerSlurm, otherwise it uses RunnerLocal.
The Runner objects are used to run computation and set singularity image. All the other task remains to the Core object.