New FSL ground truth example

This pull request introduces a new example `example002` for `FSL`. 

FSL `Example 002` (available [here](https://github.com/cmaumet/nidm/blob/fsl_ex_thresh/nidm/nidm-results/fsl/example002/fsl_nidm.ttl)) contains the minimal set of relations needed to describe a voxel-wise thresholding at p<0.05 FWE-corrected (with no thresholding on extent). This minimal set of relations is used at [incf-nidash/nidm-results_fsl](https://github.com/incf-nidash/nidm-results_fsl) to test for the correctness of the export.