WIP: Update FSL model

* Add `coordinateSpace` entities
* Add `Stats` activity
* Add `Parameter estimates` (beta maps)
* Add `Design matrix`
* Remove link between `Statistical estimate` and `Contrast estimate`
Replace `prov:atLocation` by `prov:location`
`centerOfGravity` in `fsl` namespace
Replace `nidm:hasCoordinateSpace` by `nidm:coordinateSpace`
Update png graph
Update json serialisation
Merge pull request #20 from cmaumet/fslUpdate

WIP: Update FSL model