Ground truth to be updated

[cmaumet]

This issue os open to keep a track of updates that need to be implemented for the ground truths:

• F-test and T-test should also include a 'Statistic Map' with the corresponding 'statistic Type'
• 2 t-tests ground truth
• in the thresholding ground truths, the threshold should be defined as the main threshold (i.e. used by an inference activity)
• 'spm_covariate', 'spm_2_t_tests' and 'spm_explicit_mask' are currently ignored in the test of the SPM exporter.

[cmaumet]

• Test the coordinate systems (including: subject/group/customised template)