Defintion of "noise dependence" terms

This issue is a companion for #176 to define the terms created to model noise dependence structures, namely:
- `nidm:hasNoiseDependence`
- `nidm:NoiseDependence`

( `nidm:IndependentNoise`, `nidm:CompoundSymmetryNoise`, `nidm:SeriallyCorrelatedNoise`, `nidm:ArbitrarilyCorrelatedNoise` and `nidm:ExchangeableNoise` discussed at STATO issue ISA-tools/stato#28).

Proposed definitions:
 - **nidm:hasNoiseDependence**: "Property that associates a NoiseDependence with a NoiseModel "
 - **nidm:NoiseDependence**: "Noise dependence model used to represent the dependence structure between observations used in the model parameters estimation."

Please let me know what you think. @nicholst: would you like to comment on those definitions?