Manually-crafted examples created using common template files

This is a proposal to use common template files in order to support the creation of the manually-crafted examples in `nidm/nidm-results`. 

An example of template for `DesignMatrix` is:
<pre>
<b>$design_matrix_id</b> a prov:Entity , nidm:DesignMatrix ;
	rdfs:label <b>"$label"</b> ;
	prov:atLocation <b>"$location"</b>^^xsd:anyURI ;
	dct:format <b>"$format"</b>^^xsd:string ;
	nidm:filename <b>"$filename"</b>^^xsd:string ;
	nidm:visualisation <b>$design_matrix_png_id</b> .
</pre>
Variables starting with a $ symbol are then replaced with actual values to create the examples using the scripts `create_[spm|fsl]_example_xxx.py`.

Having common templates to create the various spm and fsl examples will 
- reduce the amount of manual editing and;
- avoid inconsistency across examples when an update is made to the model.

At the same time, there is no logic included in the scripts so that those examples can still be considered as manually crafted and used for black box testing of the export tools.

The class examples created in `terms/examples` will also be useful for unit testing of the export tools.