Add RPV entity and modelling of conjunctions

Update SPM data model to include:
* Voxel per Resels map
* Modelling of conjunctions (in `spm_results_2contrasts.provn`) with a conjunction activity as an alternative to the `inference` activity
* Added a `nidm:labelId = "1";` attribute in each cluster to link with the appropriate cluster in `nidm:clusterLabels` image.