WIP: File formats as attribute

Please note that this pull request is based on #119 and #116 (PR #119 and #116 should be reviewed first).

---
As discussed in #114 and on NIDASH calls, this pull request is a first attempt at including filetypes for each file included in NIDM-Results using the Dublin Core `format` term ([dct:format](http://dublincore.org/documents/dcmi-terms/index.shtml#terms-format)).

For now this implementation is limited to `spm_results.provn`, I will extend it to all examples when we have agreed on a first set of updates.

A couple of open questions:
- Is there a preferred set of terms to use for the medical image formats?
 - There is a [ontoneurolog:NIfTI-format](http://bioportal.bioontology.org/ontologies/ONL-MR-DA/?p=classes&conceptid=http%3A%2F%2Fneurolog.unice.fr%2Fontoneurolog%2Fv3.0%2Fmedical-dataset-expression.owl%23NIfTI-format). Should we create sub-terms for `Nifti-img`, `Nifti-hdr`, `Nifti-nii` and `Nifti-nii.gz`?
 -  Or use another source for those terms?
- Do we also want to point to a term for the common format types (png, csv...) or is using as string with the MIME type ("image/png", "text/csv") sufficient (as @nicholsn in [nidm-dataset-descriptor](https://github.com/incf-nidash/nidm/blob/cdee7086ca3b18bf40570ef3cd735786b12349a5/nidm/nidm-dataset-descriptors/hcls.ttl))?
 - If we go with the terms, there seem to be a number of ontologies defining those formats (e.g. [sep:png](http://bioportal.bioontology.org/ontologies/SEP/?p=classes&conceptid=http%3A%2F%2Fpurl.obolibrary.org%2Fobo%2Fsep_00051), [hl7:png](http://bioportal.bioontology.org/ontologies/HL7?p=classes&conceptid=http%3A%2F%2Fpurl.bioontology.org%2Fontology%2FHL7%2FC1554995), [neurolog:jpg](http://bioportal.bioontology.org/ontologies/ONL-MR-DA/?p=classes&conceptid=http%3A%2F%2Fneurolog.unice.fr%2Fontoneurolog%2Fv3.0%2Fcore-ontology-programs-software.owl%23JPEG-format)).
- What do you think about the creation of an extra-attribute `nidm:headerLocation` to be used to point to the nifti `.hdr` file when `prov:atLocation` is a nifti `.img` file as in:
<pre>
entity(niiri:contrast_map_id,
      [prov:type = 'nidm:ContrastMap',
      <b>prov:location = 'localfile:Contrast.img',</b>
      <b>prov:headerLocation = 'localfile:Contrast.hdr',</b>
      ...
      ])
entity(localfile:Contrast.img,
      [prov:type = 'prov:Location',
      dct:format = 'nidm:Nifti-image'])
entity(localfile:Contrast.hdr,
      [prov:type = 'prov:Location',
      dct:format = 'nidm:Nifti-header'])     
</pre>