Non-parametric distributions

This is a proposal to add two terms to describe non-parametric distributions: 
- `non-parametric distribution`: "Probability distribution estimated empirically on the data without assumptions on the shape of the probability distribution."
- `non-parametric symmetric distribution`: "Probability distribution estimated empirically on the data assuming only symmetry of the probability distribution."

This proposal was made up with @nicholst. Definitions were also discussed with @khelm at incf-nidash/nidm#191.

A few comments:
- I was unsure whether those terms should go under `discrete probability distribution` or `continuous probability distribution`. The estimated non-parametric distribution is discrete but there is no assumption on the discreetness of the underlying true distribution. @nicholst: would you like to comment on this?
- I did not know how to set a new numerical identifier, so I used `STAT_XXXX` and `STATO_YYYY`. I am happy to update these if you can let me know how to proceed (@agbeltran, @proccaserra)?

Any comment is very welcome! Thank you.