new entity: PeakDefinitionParameters

- Create a new entity type: nidm:PeakDefinitionParameters
- maxNumberOfPeaksPerCluster and minDistanceBetweenPeaks are attributes
of nidm:PeakDefinitionParameters
new: entity ClusterConnectivityCriterion
PeakDefinitionParameters -> PeakDefinitionCriteria
nidm:ClusterConnectivityCriterion  -> nidm:ClusterDefinitionCriteria
ClusterDefinitionCriteria + terms for x-connected pixels/voxels
Update terms and examples READMEs
Merge pull request #186 from cmaumet/PeakAndClusterDefinitionParameters

... sounds good...