Add threshold value in labels for equivalent thresholds

[cmaumet]

Currently the labels of equivalent thresholds entities created with the SPM exporter is 'Height Threshold' or 'Extent Threshold' but do not provide information on the type and value of the threshold. On the other hand, the main threshold has a more detailed label (e.g. 'Height Threshold: p<0.001000 (unc.)' ).

I don't think there is any requirement on labels in the NIDM specification but maybe we could update the SPM NIDM toolbox to include the same level of details all Height/Extent thresholds?