Select i/o fields at runtime

Following one of @lhausammann's suggestion we would like to optionally select the particle fields to write at runtime by selecting them from a yaml (?) file.

The code part of it should be straightforward as we already construct such a list in all the snapshot i/o modes. We currently write all of the list, we would just have to use part of it. The slightly more tricky bit is a clean parser.

Do the same for reading in?