Definition of "noise distribution" terms

This issue is a companion for #176 to define the terms created to model noise distributions, namely:
- `nidm:hasNoiseDistribution`
- `nidm:NoiseDistribution`
- `nidm:NonParametricDistribution`
- `nidm:NonParametricSymmetricDistribution`

(`nidm:BinomialDistribution`, `nidm:GaussianDistribution` and `nidm:PoissonDistribution` are already defined as synonyms of STATO terms following a [comment by @nicholst at #176](https://github.com/incf-nidash/nidm/pull/176#issuecomment-56165837))

Proposed definition: 
 - **nidm:hasNoiseDistribution**: "Property that associates a NoiseDistribution with a NoiseModel "
 - **nidm:NoiseDistribution**: "Probability distribution used to model the residuals."
 - **nidm:NonParametricDistribution**: "Probability distribution defined empirically on the data without assumption on the shape of the probability distribution. Non-parametric distribution are usually defined using permutation of the class labels or sign-flipping."
 - **nidm:NonParametricSymmetricDistribution**: "Probability distribution defined empirically on the data assuming a symmetric probability distribution. Non-parametric distribution are usually defined using permutation of the class labels or sign-flipping."

I did not find the term "non-parametric distribution" in Bioportal (but "non-parametric test" is in [NCIT](http://bioportal.bioontology.org/ontologies/NCIT?p=classes&conceptid=http%3A%2F%2Fncicb.nci.nih.gov%2Fxml%2Fowl%2FEVS%2FThesaurus.owl%23C53229)). Once we settle on definitions, it might be worth suggesting them to STATO.

Please let me know what you think. @nicholst: would you like to comment on those definitions?