New entity for data scaling

The code in this pull request is based on the updates proposed in PR #12 ni-/ni-dm/11 ( PR #12 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 entity 'niiri:data_scaling_id' to store the underlying data scaling factor as discussed on the NIDASH-DDWG call on Decmber 2nd.