New activity `niiri:statistical_estimation_id`

The code in this pull request is based on the updates proposed in PR https://github.com/ni-/ni-dm/pull/11 ni-/ni-dm/11 ( I think that PR #11 should be reviewed before this one ).

Only the changes in examples/spm/spm_results.provn (and associated PDF) are relevant to the current PR.

New activity `niiri:statistical_estimation_id` which uses `niiri:design_matrix_id` and `niiri:contrast_id` and generates `niiri:contrast_map_id`, `niiri:contrast_variance_map_id`, `niiri:residual_sum_of_squares_map_id`, `niiri:statistical_map_id`