WIP: Excursion set and search space maps as entities

In the current data model, each entity including a nifti image is named with a term ending with `Map` (e.g. `ContrastVarianceMap`, `StatisticalMap`, ...), except for: `Excursion set` and `Search space`.

This is a proposal to create a separate:
- Excursion set map entity
- Search space map entity
- Cluster labels map entity

and then link them as attributes in the respective classes.

For example, the `Search Space` entity becomes:
<pre>
entity(niiri:search_space_id,
    [prov:type = 'nidm:SearchSpace',
    prov:label = "Search Space" %% xsd:string,
    <b>nidm:hasSearchSpaceMap = 'niiri:search_space_map_id',</b>
    spm:searchVolumeInVoxels = "65593" %% xsd:int,
    spm:searchVolumeInProductOfUnits = "1771011" %% xsd:float,
    spm:reselSize = "22.9229643140043" %% xsd:float,
    spm:searchVolumeInResels = "2552.68032521656" %% xsd:float,
    ...
   ])
<b>entity(niiri:search_space_map_id,
    [prov:type = 'nidm:SearchSpaceMap',
    prov:location = "file:///path/to/final_mask.nii" %% xsd:anyURI,
    prov:label = "Search Space Map" %% xsd:string,
    nidm:fileName = "final_mask.nii" %% xsd:string,
    nidm:coordinateSpace = 'niiri:coordinate_space_id_2',
   crypto:sha = "e43b6e01b0463fe7d40782137867a..." %% xsd:string])
</b>
</pre>
instead of:
<pre>
entity(niiri:search_space_id,
    [prov:type = 'nidm:maskMap',
    prov:label = "Search Space" %% xsd:string,
<b>    prov:location = "file:///path/to/final_mask.nii" %% xsd:anyURI,
    nidm:fileName = "final_mask.nii" %% xsd:string,
    nidm:coordinateSpace = 'niiri:coordinate_space_id_2',
    crypto:sha = "e43b6e01b0463fe7d40782137867a..." %% xsd:string</b>
    spm:searchVolumeInVoxels = "65593" %% xsd:int,
    spm:searchVolumeInProductOfUnits = "1771011" %% xsd:float,
    spm:reselSize = "22.9229643140043" %% xsd:float,
    spm:searchVolumeInResels = "2552.68032521656" %% xsd:float,
    ...
    ])
</pre>

This has the advantage to:
- group the `Map`-related attributes in an entity and;
- allow to have a different `coordinateSpace` attribute for the excursion set and the cluster labels files.

[ Please note that this does not affect current discussions regarding the renaming of `Map`. ]