Do not add binds already listed in SINGULARITY_BINDPATH. /opt bind added

following the discussion in !41 (closed)

I've removed all the --bind arguments in the wrappers (except those for GPU, since I'm not sure how to test changes in that part). The standards and additional mountpoints which are added in the wrapper are included in SINGULARITY_BINDPATH (only if they are not already in the variable). It seems to me that in this way things are a bit cleaner. But if you prefer having the explicit binds in the singularity command line, I can modify the PR accordingly.

Also /opt bind is added